fix(security): 路径穿越防护+导出行数上限+并发同步锁(3 critical/important)

- media.py: GET /api/media/{hash} 现强制 file 必须位于 Settings.media_dir
  沙箱内(resolve + relative_to),杜绝 local_path 越权读取任意文件。
- export.py: /api/export/chat-html 加 EXPORT_MAX_ROWS=5000 硬上限(.limit()),
  超出时仅渲染前 5000 条并插入 .export-warning 横幅,防止 OOM。
- scheduler.py: _safe_run 增加 SyncRun advisory lock(60min TTL),发现同
  entity 存在 status='running' 且在 TTL 内的记录即返回 skipped SyncResult;
  同时为本轮 stake 一行 wrapping SyncRun,让并发 CLI/scheduler/手动触发
  能相互避让。崩溃场景由 TTL 自动恢复。
- tests: 新增 path-traversal 拒绝/export 截断警告/advisory lock skip+TTL
  恢复 5 个测试;test_api_media 的 fixture 改为 MEDIA_DIR=tmp_path 以适配
  新沙箱语义。

Co-Authored-By: Claude <noreply@anthropic.com>

yunke-server/tests/test_api_export.py

@@ -6,8 +6,10 @@
 
 from __future__ import annotations
 
+import functools
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.api.routers.export import EXPORT_MAX_ROWS
 from app.db.models import Message
 
 
@@ -39,6 +41,26 @@ async def _seed(session: AsyncSession) -> None:
     await session.commit()
 
 
+async def _seed_many(session: AsyncSession, n: int) -> None:
+    """Seed ``n`` DM messages so the row cap can be exercised."""
+    session.add_all([
+        Message(
+            msg_id=f"cap{i}",
+            channel="wechat",
+            owner_account_id="wx_owner",
+            talker_id="wx_friend",
+            room_id=None,
+            text=f"msg{i}",
+            timestamp=1000 + i,
+            is_send=i % 2 == 0,
+            type="text",
+            type_raw=1,
+        )
+        for i in range(n)
+    ])
+    await session.commit()
+
+
 async def test_export_dm_returns_html(make_client):
     client = await make_client(_seed)
     r = await client.get("/api/export/chat-html", params={
@@ -83,3 +105,35 @@ async def test_export_embeds_media(make_client):
     # 本地路径 → 占位提示,不嵌 <audio>
     assert "[本地媒体]" in body
     assert "/api/media/abc123" not in body
+
+
+async def test_export_caps_rows_and_emits_warning(make_client):
+    """When the cap is hit, only EXPORT_MAX_ROWS rows render + a warning shows."""
+    # Seed twice the cap so the truncation is unambiguous.
+    seed = functools.partial(_seed_many, n=EXPORT_MAX_ROWS * 2)
+    client = await make_client(seed)
+    r = await client.get("/api/export/chat-html", params={
+        "channel": "wechat", "owner_account_id": "wx_owner", "talker_id": "wx_friend"})
+    assert r.status_code == 200
+    body = r.text
+    # The warning DIV is rendered (not just the CSS class in <style>).
+    assert '<div class="export-warning">' in body
+    assert str(EXPORT_MAX_ROWS) in body
+    # The first EXPORT_MAX_ROWS seeded rows (timestamps 1000..1000+N-1) render;
+    # anything past the cap must NOT appear. msg_id pattern "cap{i}" lets us
+    # count rendered bubbles precisely.
+    last_kept = EXPORT_MAX_ROWS - 1
+    last_dropped = EXPORT_MAX_ROWS  # first row beyond the cap
+    assert f"msg{last_kept}" in body
+    assert f"msg{last_dropped}" not in body
+
+
+async def test_export_no_warning_below_cap(make_client):
+    """Sub-cap exports render no warning DIV."""
+    seed = functools.partial(_seed_many, n=10)
+    client = await make_client(seed)
+    r = await client.get("/api/export/chat-html", params={
+        "channel": "wechat", "owner_account_id": "wx_owner", "talker_id": "wx_friend"})
+    assert r.status_code == 200
+    # No warning DIV (the CSS class still lives in <style> — that's fine).
+    assert '<div class="export-warning">' not in r.text