feat(wechat): 微信里发一条链接，正文和配图抓下来存成知识

公众号的文章也能抓：走 mp.weixin.qq.com 的 DOM（#js_content），
按 data-src 取图，不靠通用抽取器——它会把公众号的图全丢掉。

正文里的外链图下载进我们自己的存储（最多 12 张、单张 8 MB），
地址改成 /attachments/{id}/raw?签名：公众号那边一换图，外链就是一排红叉。

抓取入口谁都能触发，所以只允许公网地址（内网段、回环、链路本地、
保留段、解析不出的域名一律拒绝），否则就是个 SSRF 跳板。

抓不到要说人话："这个链接没抓下来（原因）。想直接记成任务的话，
加「任务：」前缀再发一次。" 反爬验证页、SPA 空壳都走这条路。

- 新增 services/article.py：extract_url / parse / fetch / store_images / compose_body
- wechat_intake：整条消息是链接时走抓取，落知识草稿，回「已存成知识草稿：标题」
- 依赖 trafilatura==2.0.0（装法见 requirements.txt 注释：必须逐个装再 --no-deps）
- 测试 9 例：链接判定、公众号解析、验证页、通用页裁导航、SSRF 拒绝、图片本地化、失败提示

backend/app/services/article.py

@@ -0,0 +1,513 @@
+"""把一条链接抓成一篇知识。
+
+微信里发一条链接，多半就是想"把这篇文章存下来"。这里负责三件事：
+
+1. **取正文**：用 trafilatura 的正文抽取（开源社区里在中文站上表现最稳的一个），
+   微信公众号单独走一条路——它的正文在 `#js_content` 里、图片放在 `data-src`，
+   通用抽取器会把图丢掉。
+2. **留来源**：正文开头记一行原文地址与作者，日后要核对时不用猜。
+3. **图片转存**：文章里的图片不是躺在我们的存储里，公众号一改就变红叉。
+   所以抓下来的图逐张存成附件，正文里的地址换成我们自己的签名直链。
+
+抓不下来不硬扛：调用方拿到 `ArticleError` 就把原链接当普通文本存成任务，
+用户至少不会丢东西。
+"""
+
+from __future__ import annotations
+
+import ipaddress
+import re
+import socket
+import uuid
+from dataclasses import dataclass, field
+from urllib.parse import urlparse
+
+import httpx
+from sqlalchemy.ext.asyncio import AsyncSession
+from starlette.concurrency import run_in_threadpool
+
+# 先让 app.models 加载完：它在文件末尾反向导入各域的租户表清单，
+# 直接 import models_attachment 会撞上"部分初始化"的循环导入。
+from .. import models as _models  # noqa: F401
+from ..config import get_settings
+from ..models_attachment import Attachment
+from . import signing, storage
+
+# 一条消息里只认第一个链接；带别的字就当普通任务（"看下 https://… 这个" 不该被吞成收藏）
+URL_RE = re.compile(r"https?://[^\s<>\"'）)】。，；]+", re.I)
+
+MAX_ARTICLES_BYTES = 8 * 1024 * 1024
+MAX_IMAGES = 12
+MAX_IMAGE_BYTES = 8 * 1024 * 1024
+MAX_BODY_CHARS = 200_000
+FETCH_TIMEOUT = 25.0
+
+DESKTOP_UA = (
+    "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36"
+    " (KHTML, like Gecko) Chrome/126.0 Safari/537.36"
+)
+# 公众号对"非微信浏览器"会压缩内容、图片懒加载，用微信 UA 拿到的才是完整一篇。
+WECHAT_UA = (
+    "Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15"
+    " (KHTML, like Gecko) Mobile/15E148 MicroMessenger/8.0.44(0x18002c2e)"
+    " NetType/WIFI Language/zh_CN"
+)
+
+IMAGE_EXTS = {"jpeg": "jpg", "jpg": "jpg", "png": "png", "gif": "gif", "webp": "webp"}
+
+
+class ArticleError(RuntimeError):
+    """抓取/解析没成功。调用方据此决定降级方式。"""
+
+
+@dataclass
+class Article:
+    url: str
+    title: str
+    body_md: str
+    site: str = ""
+    author: str = ""
+    images: list[str] = field(default_factory=list)
+
+
+def extract_url(text: str) -> str | None:
+    """这一整句话就是个链接吗？
+
+    允许链接前后有点标点、空格（"看下：https://x" 不算）；带了别的内容就当普通文本，
+    免得用户随口提一句网址，心情是"记条任务"，结果被存成一篇文章。
+    """
+    stripped = text.strip()
+    match = URL_RE.search(stripped)
+    if not match:
+        return None
+    rest = (stripped[: match.start()] + stripped[match.end() :]).strip()
+    rest = rest.strip(" \t\r\n:：，。；;、!！?？（）()【】[]<>「」")
+    if rest:
+        return None
+    return match.group(0)
+
+
+def is_wechat_article(url: str) -> bool:
+    host = (urlparse(url).hostname or "").lower()
+    return host == "mp.weixin.qq.com"
+
+
+async def _assert_public(url: str) -> None:
+    """只抓公网地址。
+
+    这个入口能由工作区以外的人（微信里的任何人）触发，放任它就是一台 SSRF 跳板：
+    `http://127.0.0.1:8200/version`、`http://169.254.169.254/` 之类都能被打。
+    """
+    parsed = urlparse(url)
+    if parsed.scheme not in ("http", "https"):
+        raise ArticleError("只支持 http/https 链接")
+    host = parsed.hostname or ""
+    if not host:
+        raise ArticleError("链接里没有主机名")
+    try:
+        infos = await _resolve(host)
+    except OSError as exc:  # 解析不出来就别猜
+        raise ArticleError(f"解析不了这个域名：{exc}") from exc
+    for _family, _type, _proto, _canon, sockaddr in infos:
+        ip = ipaddress.ip_address(sockaddr[0])
+        if (
+            ip.is_private
+            or ip.is_loopback
+            or ip.is_link_local
+            or ip.is_reserved
+            or ip.is_multicast
+            or ip.is_unspecified
+        ):
+            raise ArticleError("这个地址在内网，不抓")
+
+
+async def _resolve(host: str):
+    import asyncio
+
+    def lookup():
+        return socket.getaddrinfo(host, None)
+
+    return await asyncio.to_thread(lookup)
+
+
+def _pick_ua(url: str) -> str:
+    return WECHAT_UA if is_wechat_article(url) else DESKTOP_UA
+
+
+def _looks_blocked(html: str) -> bool:
+    probes = (
+        "环境异常",
+        "完成验证后即可继续访问",
+        "请在微信客户端打开链接",
+        "该内容已被发布者删除",
+        "此内容因违规无法查看",
+    )
+    return any(p in html for p in probes)
+
+
+def _clean_text(value: str) -> str:
+    return re.sub(r"[ \t\u00a0]+", " ", (value or "").replace("\r", "")).strip()
+
+
+def _wechat_meta(html: str) -> tuple[str, str, list[tuple[str, str]]]:
+    """从公众号页面里取 (标题, 作者/公众号名, [(图片地址, alt)]) 与正文节点。"""
+    from lxml import html as lxml_html
+
+    tree = lxml_html.fromstring(html)
+    title = ""
+    for xpath in (
+        '//*[@id="activity-name"]//text()',
+        '//meta[@property="og:title"]/@content',
+        "//title/text()",
+    ):
+        parts = [_clean_text(t) for t in tree.xpath(xpath)]
+        title = " ".join(p for p in parts if p)
+        if title:
+            break
+    author = ""
+    for xpath in (
+        '//*[@id="js_name"]//text()',
+        '//meta[@name="author"]/@content',
+        '//*[@id="meta_content"]//*[@class="rich_media_meta_text"]//text()',
+    ):
+        parts = [_clean_text(t) for t in tree.xpath(xpath)]
+        author = " ".join(p for p in parts if p)
+        if author:
+            break
+    return title, author, tree
+
+
+def _wechat_body(html: str) -> str:
+    """`#js_content` → markdown。
+
+    公众号正文是一堆嵌套 `section` 拼版，段落文字常被拆成多个 `span`；
+    这里只取"没有块级子节点"的元素，避免同一个句子被输出好几遍。
+    """
+    from lxml import html as lxml_html
+
+    tree = lxml_html.fromstring(html)
+    nodes = tree.xpath('//*[@id="js_content"]')
+    if not nodes:
+        return ""
+    content = nodes[0]
+
+    block_tags = {
+        "p",
+        "section",
+        "div",
+        "blockquote",
+        "li",
+        "h1",
+        "h2",
+        "h3",
+        "h4",
+        "h5",
+        "h6",
+        "pre",
+    }
+    lines: list[str] = []
+    for node in content.iter():
+        tag = node.tag if isinstance(node.tag, str) else ""
+        if tag == "img":
+            src = node.get("data-src") or node.get("src") or ""
+            if src.startswith("http"):
+                lines.append(f"![]({src})")
+            continue
+        if tag == "br":
+            lines.append("")
+            continue
+        if tag not in block_tags:
+            continue
+        # 还有块级子孙的容器交给子节点输出，自己不出文字，否则整段重复
+        if any(
+            (child.tag if isinstance(child.tag, str) else "") in block_tags
+            for child in node.iterdescendants()
+        ):
+            continue
+        text = _clean_text("".join(node.itertext()))
+        if not text:
+            continue
+        if tag.startswith("h") and len(tag) == 2:
+            text = f"## {text}"
+        elif tag == "li":
+            text = f"- {text}"
+        elif tag == "blockquote":
+            text = f"> {text}"
+        lines.append(text)
+
+    # 相邻空行压成一个，段间留空行
+    out: list[str] = []
+    for line in lines:
+        if not line and (not out or not out[-1]):
+            continue
+        out.append(line)
+    return "\n\n".join(s.strip() for s in "\n".join(out).split("\n\n") if s.strip())
+
+
+def _block_markdown(html: str) -> str:
+    """兜底的正文提取：找出"装着最多段文字"的那个容器，再按段落拼成 markdown。
+
+    为什么还要这一手：trafilatura 在结构清楚的资讯站上很准，但遇到"整页一个 div、
+    导航和正文混在一起"的老站点（政府站、企业站）时会连导航一起端出来。
+    这里用最笨但稳的判据——哪个容器里的 <p> 文字最多，正文就在哪个容器里。
+    """
+    from lxml import html as lxml_html
+
+    tree = lxml_html.fromstring(html)
+    best = None
+    best_len = 0
+    for node in tree.iter("div", "article", "section", "main", "td"):
+        total = sum(len("".join(p.itertext()).strip()) for p in node.iter("p"))
+        if total > best_len:
+            best, best_len = node, total
+    if best is None or best_len < 200:
+        return ""
+
+    parts: list[str] = []
+    for node in best.iter():
+        tag = node.tag if isinstance(node.tag, str) else ""
+        if tag == "img":
+            src = node.get("data-src") or node.get("src") or ""
+            if src.startswith("http"):
+                parts.append(f"![]({src})")
+            continue
+        if tag not in {"p", "h1", "h2", "h3", "h4", "blockquote", "li"}:
+            continue
+        text = _clean_text("".join(node.itertext()))
+        if not text:
+            continue
+        if tag.startswith("h") and len(tag) == 2:
+            text = f"## {text}"
+        elif tag == "li":
+            text = f"- {text}"
+        elif tag == "blockquote":
+            text = f"> {text}"
+        parts.append(text)
+    return "\n\n".join(parts).strip()
+
+
+def _body_score(body: str) -> int:
+    """给一份抽取结果打分：有多少段"像人写的段落"。
+
+    只用来在两个候选之间挑一个，不需要绝对准确——比较的是同一篇文章的两种切法。
+    """
+    lines = [line.strip() for line in body.splitlines() if line.strip()]
+    return sum(1 for line in lines if len(line) >= 40)
+
+
+def _trim_chrome(body: str) -> str:
+    """去掉抽取器一并带出来的导航与页脚。
+
+    按行看，找出第一行和最后一行"像正文的"（够长、或者有句读），把两头那些短标签
+    （"Java教程"、"Sign In"）切掉。按行而不是按段落，是因为导航往往是一行一个、
+    中间不空行，按段落切会把它和正文粘成一整块，判不出边界。
+    """
+    lines = body.splitlines()
+    if not lines:
+        return ""
+
+    def substantive(line: str) -> bool:
+        text = line.strip()
+        if not text or text.startswith(("![", "> ")):
+            return False
+        plain = text.lstrip("#-* 　")
+        return len(plain) >= 20 or bool(re.search(r"[。！？；]", plain))
+
+    first = next((i for i, line in enumerate(lines) if substantive(line)), None)
+    if first is None:
+        return body.strip()
+    last = max(i for i, line in enumerate(lines) if substantive(line))
+    # 图片通常紧贴在正文前后，边界外紧邻的图保留
+    while first > 0 and lines[first - 1].strip().startswith("!["):
+        first -= 1
+    while last + 1 < len(lines) and lines[last + 1].strip().startswith("!["):
+        last += 1
+    return "\n".join(lines[first : last + 1]).strip()
+
+
+def _generic_article(html: str, url: str) -> Article:
+    """非公众号：交给 trafilatura，它在正文/噪声的判别上比手搓规则靠谱得多。
+
+    先按 `favor_precision` 抠最干净的一块；它经常因为"看着都不像正文"而返回空，
+    再退回默认召回模式。两头再削一刀导航。
+    """
+    import trafilatura
+
+    metadata = trafilatura.extract_metadata(html, default_url=url)
+    common = {
+        "url": url,
+        "output_format": "markdown",
+        "include_images": True,
+        "include_links": True,
+        "include_formatting": True,
+    }
+    body = trafilatura.extract(html, favor_precision=True, **common) or ""
+    if len(body.strip()) < 120:
+        body = trafilatura.extract(html, **common) or ""
+    if not body.strip():
+        # 再退一步：trafilatura 认不出结构时，退到纯文本
+        body = trafilatura.extract(html, url=url, output_format="txt") or ""
+    body = _trim_chrome(body)
+
+    # 老站点（政府站、企业站）常把导航和正文塞进同一个 div，trafilatura 会连导航一起端出来。
+    # 用"哪个容器里段落文字最多"另取一份，两份按"像样段落的条数"挑更干净的那个。
+    block = _trim_chrome(_block_markdown(html))
+    if _body_score(block) > _body_score(body):
+        body = block
+    if not body.strip():
+        raise ArticleError("这个页面里没找到正文（可能是动态渲染或需要登录）")
+    title = _clean_text(getattr(metadata, "title", "") or "")
+    return Article(
+        url=url,
+        title=title,
+        body_md=body.strip(),
+        site=_clean_text(getattr(metadata, "sitename", "") or ""),
+        author=_clean_text(getattr(metadata, "author", "") or ""),
+    )
+
+
+def parse_article(html: str, url: str) -> Article:
+    if _looks_blocked(html):
+        raise ArticleError("微信把这篇拦下来了（需要人工验证或链接已失效）")
+    if is_wechat_article(url):
+        title, author, _tree = _wechat_meta(html)
+        body = _wechat_body(html)
+        if not body.strip():
+            # 公众号偶尔会换结构，退回通用抽取，宁可少图也别丢正文
+            article = _generic_article(html, url)
+            article.author = author or article.author
+            return article
+        return Article(
+            url=url,
+            title=title,
+            body_md=body.strip(),
+            site="微信公众号",
+            author=author,
+        )
+    return _generic_article(html, url)
+
+
+async def fetch_article(url: str) -> Article:
+    await _assert_public(url)
+    headers = {
+        "User-Agent": _pick_ua(url),
+        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
+        "Accept-Language": "zh-CN,zh;q=0.9,en;q=0.6",
+    }
+    try:
+        async with httpx.AsyncClient(
+            follow_redirects=True, timeout=FETCH_TIMEOUT, headers=headers
+        ) as client:
+            resp = await client.get(url)
+            resp.raise_for_status()
+            raw = resp.content
+    except httpx.HTTPStatusError as exc:
+        raise ArticleError(f"对方站点返回 {exc.response.status_code}") from exc
+    except httpx.HTTPError as exc:
+        raise ArticleError(f"取不到这个页面：{type(exc).__name__}") from exc
+    if len(raw) > MAX_ARTICLES_BYTES:
+        raise ArticleError("页面太大了，不像是篇文章")
+    html = raw.decode(resp.encoding or "utf-8", errors="replace")
+    article = parse_article(html, url)
+    article.title = (article.title or urlparse(url).hostname or "未命名")[:180]
+    return article
+
+
+async def _download_image(client: httpx.AsyncClient, url: str) -> tuple[bytes, str] | None:
+    try:
+        resp = await client.get(url)
+        resp.raise_for_status()
+    except httpx.HTTPError:
+        return None
+    if len(resp.content) > MAX_IMAGE_BYTES:
+        return None
+    ctype = (resp.headers.get("content-type") or "").split(";")[0].strip().lower()
+    if not ctype.startswith("image/"):
+        # 公众号图片地址带 wx_fmt 参数，content-type 偶尔是 octet-stream，按扩展名兜一下
+        fmt = urlparse(url).query or ""
+        m = re.search(r"wx_fmt=(\w+)", fmt)
+        guess = IMAGE_EXTS.get((m.group(1) if m else "").lower(), "")
+        if not guess:
+            return None
+        ctype = f"image/{'jpeg' if guess == 'jpg' else guess}"
+    return resp.content, ctype
+
+
+async def store_images(
+    session: AsyncSession,
+    *,
+    ws_id: uuid.UUID,
+    item_id: uuid.UUID,
+    uploader_id: uuid.UUID,
+    body_md: str,
+    limit: int = MAX_IMAGES,
+) -> tuple[str, list[Attachment]]:
+    """把正文里引用的远程图片存成我们自己的附件，返回改写后的正文与附件行。
+
+    失败就当没这张图：一张打不开的配图不该让整篇收藏失败。
+    """
+    from io import BytesIO
+
+    urls = re.findall(r"!\[[^\]]*\]\((https?://[^)\s]+)\)", body_md)
+    unique: list[str] = []
+    for url in urls:
+        if url not in unique:
+            unique.append(url)
+        if len(unique) >= limit:
+            break
+    if not unique:
+        return body_md, []
+
+    settings = get_settings()
+    saved: list[Attachment] = []
+    mapping: dict[str, str] = {}
+    headers = {"User-Agent": DESKTOP_UA, "Referer": "https://mp.weixin.qq.com/"}
+    async with httpx.AsyncClient(
+        follow_redirects=True, timeout=FETCH_TIMEOUT, headers=headers
+    ) as client:
+        for index, remote in enumerate(unique, start=1):
+            got = await _download_image(client, remote)
+            if got is None:
+                continue
+            payload, content_type = got
+            ext = IMAGE_EXTS.get(content_type.split("/")[-1], "jpg")
+            # 落盘是阻塞 IO：一篇公众号十几张图，别把事件循环按住
+            stored = await run_in_threadpool(
+                storage.save, BytesIO(payload), workspace_id=ws_id, item_id=item_id
+            )
+            if stored.size > settings.max_upload_mb * 1024 * 1024:
+                storage.remove(stored.key)
+                continue
+            row = Attachment(
+                workspace_id=ws_id,
+                item_id=item_id,
+                uploader_id=uploader_id,
+                filename=f"图{index}.{ext}",
+                content_type=content_type[:120],
+                size_bytes=stored.size,
+                sha256=stored.sha256,
+                storage_key=stored.key,
+            )
+            session.add(row)
+            await session.flush()
+            mapping[remote] = (
+                f"{settings.api_prefix}/workspaces/{ws_id}"
+                f"/attachments/{row.id}/raw?{signing.sign(ws_id, row.id)}"
+            )
+            saved.append(row)
+
+    for remote, local in mapping.items():
+        body_md = body_md.replace(remote, local)
+    return body_md, saved
+
+
+def compose_body(article: Article) -> str:
+    """正文 = 来源行 + 原文。来源要留在最上面，日后核对不用翻聊天记录。"""
+    bits = [f"> 原文：<{article.url}>"]
+    if article.site or article.author:
+        who = " · ".join(x for x in (article.site, article.author) if x)
+        bits[0] += f"（{who}）"
+    body = "\n".join(bits) + "\n\n" + article.body_md
+    if len(body) > MAX_BODY_CHARS:
+        body = body[:MAX_BODY_CHARS] + "\n\n…（原文太长，后面的内容没有保存）"
+    return body