feat(rich): 安卓能编辑富文本了，图片直链换成短期签名，表格不再撑破抽屉

三件欠账一次清：

1. 图片直链签名化
   - 新增 `backend/app/services/signing.py`：HMAC-SHA256，密钥由 `JWT_SECRET` 域分离派生
     （`attachment-raw-v1`），载荷 `workspace_id:attachment_id:exp`，默认 30 分钟
   - `/raw` 支持 `?exp=&sig=`，不需要登录态；签名里绑了工作区，跨租户链接直接拒
   - 附件对象多一个 `raw_url`，前端每次渲染现签，正文里只存不带凭据的 `/raw`
   - 保留旧 `?token=` 通道，给未升级的客户端过渡

2. 安卓富文本编辑器
   - `ui/RichTextEditorScreen.kt`：Markdown 工具栏（粗体/斜体/标题/列表/引用/代码/链接/图片），
     选区包裹与行首前缀都按光标位置来；贴图走附件接口，插入签名直链
   - 预览用 Markwon，图片用 Coil；正文落在一张白纸卡片里，不再贴屏幕边
   - 从任务详情与知识点都能进，保存按 `body_md` 回写（PATCH 带 version）

3. 表格
   - 只读渲染走 markdown-it 的 GFM 表，`display:block + overflow-x:auto`：
     宽表横向滚动、窄表铺满，都不撑破抽屉

验证：pytest 74 passed；richtext 15 / smoke 7 / tickets 8 / knowledge 7 全绿；
tsc -b 0 错；vite build 通过（主包 299.42 kB / gzip 74.96 kB）；
安卓 assembleDebug 成功并在 API 35 模拟器走通编辑与预览。

仍欠：保存后回列表而非回详情抽屉、图片压缩与缩略图、HTML 片段按文本显示。

backend/tests/test_attachments.py

@@ -13,7 +13,10 @@ from typing import Any
 import pytest
 from httpx import AsyncClient
 
+import uuid
+
 from app.config import get_settings
+from app.services import signing
 
 from .conftest import create_workspace, login, unwrap
 
@@ -160,11 +163,66 @@ async def test_raw_image_url_takes_token_in_query(client: AsyncClient) -> None:
     assert resp.headers["content-disposition"] == "inline"
     assert resp.headers["cache-control"].startswith("private")
 
-    # 不带 token 直接 422（参数必填），带了别人的 token 一律当不存在
+    # 既没签名也没令牌：401（而不是把"缺凭据"说成"文件不存在"）
     missing = await client.get(f"{BASE}/workspaces/{ws['id']}/attachments/{created['id']}/raw")
-    assert missing.status_code == 422
+    assert missing.status_code == 401
+    assert missing.json()["code"] == 2001
     outsider = await login(client)
     leaked = await client.get(
         f"{BASE}/workspaces/{ws['id']}/attachments/{created['id']}/raw?token={outsider['token']}"
     )
     assert leaked.json()["code"] == 8001
+
+
+async def test_raw_url_uses_short_lived_signature(client: AsyncClient) -> None:
+    """图片直链自带短期签名：不需要把登录令牌贴进 URL，也不需要 Authorization 头。"""
+    user, ws, item_id = await _ws_with_item(client)
+    body = b"\x89PNG\r\n\x1a\n signed png"
+    created = unwrap(await _upload(client, user, ws["id"], item_id, body, "签名图.png"))
+
+    listed = unwrap(
+        await client.get(
+            f"{BASE}/workspaces/{ws['id']}/items/{item_id}/attachments", headers=user["headers"]
+        )
+    )
+    raw_url = listed["items"][0]["raw_url"]
+    assert "exp=" in raw_url and "sig=" in raw_url
+    assert "token=" not in raw_url  # 不该再把登录令牌塞进 URL
+
+    # 裸请求（无 Authorization、无 cookie）也要能取到图——这才是 <img> 的真实处境
+    resp = await client.get(raw_url)
+    assert resp.status_code == 200
+    assert resp.content == body
+
+
+async def test_raw_url_rejects_tampered_or_expired_signature(client: AsyncClient) -> None:
+    user, ws, item_id = await _ws_with_item(client)
+    created = unwrap(await _upload(client, user, ws["id"], item_id, b"data", "图.png"))
+    base = f"{BASE}/workspaces/{ws['id']}/attachments/{created['id']}/raw"
+
+    # 改一个字符就该失效：签名不是"看起来对就行"
+    good = signing.sign(uuid.UUID(ws["id"]), uuid.UUID(created["id"]))
+    tampered = good[:-1] + ("0" if good[-1] != "0" else "1")
+    bad = await client.get(f"{base}?{tampered}")
+    assert bad.status_code == 403
+    assert bad.json()["code"] == 8003
+
+    # 过期签名：就算 HMAC 算得对，时间过了也不给
+    stale = signing.sign(uuid.UUID(ws["id"]), uuid.UUID(created["id"]), ttl_seconds=-10)
+    expired = await client.get(f"{base}?{stale}")
+    assert expired.status_code == 403
+    assert expired.json()["code"] == 8003
+
+
+async def test_raw_url_signature_is_workspace_scoped(client: AsyncClient) -> None:
+    """拿 A 工作区签出来的链接去请求 B 工作区，必须失败（签名里带 workspace_id）。"""
+    user, ws_a, item_a = await _ws_with_item(client, "甲工作区")
+    ws_b = await create_workspace(client, user, "乙工作区")
+    created = unwrap(await _upload(client, user, ws_a["id"], item_a, b"data", "图.png"))
+
+    query = signing.sign(uuid.UUID(ws_a["id"]), uuid.UUID(created["id"]))
+    crossed = await client.get(
+        f"{BASE}/workspaces/{ws_b['id']}/attachments/{created['id']}/raw?{query}"
+    )
+    assert crossed.status_code == 403
+    assert crossed.json()["code"] == 8003