fix(chat): resolve Auto-route failover event-loop freeze (#812)

Root cause: ensure_structured_column() executed a synchronous blocking
engine.connect().execute(SELECT information_schema.columns ...) inside the
async request path (_build_chat_history / _maybe_compress_history), freezing
the uvicorn event loop and hanging all endpoints (~200s, recovered only by
docker restart).

Fix:
- chat_context.py: remove per-request ensure_structured_column() calls; run
  once idempotently at app startup (main.py lifespan). Use CURRENT_DATABASE()
  for PostgreSQL compatibility.
- main.py: add startup-only ensure_structured_column() hook (try/except).
- llm_client.py: trust_env=False + bounded httpx.Timeout/wait_for so broken
  endpoints fail fast instead of hanging the event loop.
- chat_service.py: run media generation in a background daemon thread with its
  own event loop (_launch_media_task_bg) so SSE end does not cancel image/video
  tasks (#811); remove TEMP faulthandler diagnostic.

Safe-align: persist accumulated live container changes (admin/*, knowledge,
market, models, config, observability, mcp_client, platform_knowledge,
im_bot_platform, chat_media, capability_registry, requirements) and add
workflow_templates.

Verified: live failover test (model-006 -> conn-refused) returns in ~1.9s with
concurrent login 0.83s/200; event-loop responsiveness restored (sub-second).

backend/python/app/api/market.py

@@ -59,7 +59,7 @@ def _sanitize_skill_content(text: str) -> str:
     if "<script" in low and t.count("<div") > 3:
         return ""
     # 接受携带任一 markdown / skill 标记的内容
-    markers = ("#", "---", "name:", "description:", "```", "##")
+    markers = ("#", "---", "name:", "description:", "\"\"\"", "##")
     if not any(m in t for m in markers):
         return ""
     return t
@@ -79,7 +79,7 @@ def _extract_skill_from_html(html: str, base_url: str) -> str:
     """从 HTML 页面尽力抽取 SKILL.md 正文（代码块优先，其次跳转到 raw 链接）。"""
     proxy = _skill_http_proxy()
     # 1) fenced code block
-    fences = re.findall(r"```(?:markdown|md|yaml|json)?\s*\n(.*?)```", html, re.DOTALL | re.IGNORECASE)
+    fences = re.findall(r"\"\"\"(?:markdown|md|yaml|json)?\s*\n(.*?)\"\"\"", html, re.DOTALL | re.IGNORECASE)
     for f in fences:
         if ("name:" in f.lower() or "description:" in f.lower() or f.count("#") >= 2) and len(f) > 80:
             return f.strip()