fix(wechat): 公众号新版页面抓不到正文，改从 cgiDataNew 里读

用户发来 https://mp.weixin.qq.com/s/ZEOsi4R4JcmZ_mhS6qpcww，存进库的正文只有 67 个字符
（"原文链接 + 一个句号"）。复盘发现公众号换了渲染方式：新版页面把整篇塞进
window.cgiDataNew 这个 JS 对象由前端渲染，页面里根本没有 id="js_content"；
三种 UA 拿到的字节数完全一样，也不是 UA 的问题。

- 从 JS 字符串字面量里读正文（content_noencode / desc），按 \xNN、\uNNNN 反转义，
  话题标签是双重转义过的 HTML，先 unescape 再走 DOM。
- 配图字段排在文字之后，所以前后各找 2500 字符合图；剩下没用过的 mmbiz 图按
  12 张上限补在末尾（图文型文章靠就近配对会漏）。
- 解析顺序：旧 DOM → 新版 JS 数据 → 通用抽取，一层层退。

实测两篇：2373 字符 + 12 张图、2772 字符 + 13 张图，作者也都取到了。

同时补上用户提的两件：

- 知识能删了：DELETE /knowledge/{id}?confirm=true，软删 + 真删检索切片，
  只有作者或 owner/admin 能删；列表卡片（悬停出现）和阅读页动作行各一个入口。
  删完详情也回 7001，否则收藏的链接还能打开一篇已删除的知识。
- 分类下面有标签清单了：GET /knowledge/tags 按标签数"有多少条知识用了它"，
  侧栏渲染成 #标签 + 条数，点一下筛选、再点取消。

backend/tests/test_knowledge.py

@@ -423,3 +423,118 @@ async def test_list_pages_through_every_document(client: AsyncClient) -> None:
     assert cursor is None
     assert sorted(seen) == titles
     assert len(seen) == len(set(seen)), "翻页翻出了重复条目"
+
+
+async def test_delete_removes_from_list_and_search(client: AsyncClient) -> None:
+    """删掉的知识不能还留在列表和检索里——切片要一起删。"""
+    owner = await login(client)
+    ws = await create_workspace(client, owner, "删除知识")
+    note = unwrap(
+        await client.post(
+            f"{V1}/workspaces/{ws['id']}/knowledge",
+            json={
+                "title": "回滚前先停灰度",
+                "body_md": "灰度流量上涨后错误率飙升，第一步先停灰度。",
+                "tag_names": ["发布", "回滚"],
+            },
+            headers=owner["headers"],
+        )
+    )
+    unwrap(
+        await client.post(
+            f"{V1}/workspaces/{ws['id']}/knowledge/{note['id']}/status",
+            json={"action": "publish", "version": note["version"]},
+            headers=owner["headers"],
+        )
+    )
+
+    # 不带 confirm 不给删，防手滑
+    resp = await client.delete(
+        f"{V1}/workspaces/{ws['id']}/knowledge/{note['id']}", headers=owner["headers"]
+    )
+    assert resp.json()["code"] == 1002
+
+    unwrap(
+        await client.delete(
+            f"{V1}/workspaces/{ws['id']}/knowledge/{note['id']}",
+            params={"confirm": "true"},
+            headers=owner["headers"],
+        )
+    )
+
+    page = unwrap(
+        await client.get(f"{V1}/workspaces/{ws['id']}/knowledge", headers=owner["headers"])
+    )
+    assert page["items"] == []
+    hits = unwrap(
+        await client.get(
+            f"{V1}/workspaces/{ws['id']}/knowledge/search",
+            params={"q": "灰度"},
+            headers=owner["headers"],
+        )
+    )
+    assert hits["items"] == []
+    tags = unwrap(
+        await client.get(f"{V1}/workspaces/{ws['id']}/knowledge/tags", headers=owner["headers"])
+    )
+    assert tags["items"] == [], "删掉的知识不该再给标签贡献条数"
+
+    # 删完再从收藏的链接点进来：要当它不存在，不能还渲染出一篇"删掉了的知识"
+    detail = await client.get(
+        f"{V1}/workspaces/{ws['id']}/knowledge/{note['id']}", headers=owner["headers"]
+    )
+    assert detail.json()["code"] == 7001
+    again = await client.delete(
+        f"{V1}/workspaces/{ws['id']}/knowledge/{note['id']}",
+        params={"confirm": "true"},
+        headers=owner["headers"],
+    )
+    assert again.json()["message"] == "已经删过了", "重复删不该报错，回一句就够"
+
+
+async def test_only_author_or_admin_can_delete(client: AsyncClient) -> None:
+    owner = await login(client)
+    ws = await create_workspace(client, owner, "谁能删")
+    member = await _member(client, owner, ws)
+    note = unwrap(
+        await client.post(
+            f"{V1}/workspaces/{ws['id']}/knowledge",
+            json={"title": "别人的笔记", "body_md": "随手记的"},
+            headers=owner["headers"],
+        )
+    )
+
+    resp = await client.delete(
+        f"{V1}/workspaces/{ws['id']}/knowledge/{note['id']}",
+        params={"confirm": "true"},
+        headers=member["headers"],
+    )
+    assert resp.json()["code"] == 3004
+
+    # 作者自己能删
+    unwrap(
+        await client.delete(
+            f"{V1}/workspaces/{ws['id']}/knowledge/{note['id']}",
+            params={"confirm": "true"},
+            headers=owner["headers"],
+        )
+    )
+
+
+async def test_tags_endpoint_counts_documents(client: AsyncClient) -> None:
+    owner = await login(client)
+    ws = await create_workspace(client, owner, "标签统计")
+    for title, tags in (("甲", ["发布", "回滚"]), ("乙", ["发布"])):
+        unwrap(
+            await client.post(
+                f"{V1}/workspaces/{ws['id']}/knowledge",
+                json={"title": title, "tag_names": tags},
+                headers=owner["headers"],
+            )
+        )
+
+    got = unwrap(
+        await client.get(f"{V1}/workspaces/{ws['id']}/knowledge/tags", headers=owner["headers"])
+    )
+    counts = {row["name"]: row["count"] for row in got["items"]}
+    assert counts == {"发布": 2, "回滚": 1}