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/capability_registry.py

@@ -98,10 +98,11 @@ def resolve_auto_model(capability: str, db: Session) -> Optional[AIModel]:
     # 媒体类能力(image/video/audio)：无显式配置时自动发现任一启用的模型
     # 避免用户未在后台配置"能力默认模型"时 Auto 回退到文本模型导致"无法生成图片/视频"
     if capability in ("image", "video", "audio"):
-        from sqlalchemy import text as _text, func
+        from sqlalchemy import text as _text, func, type_coerce
+        from sqlalchemy.dialects.postgresql import JSONB
         m = db.query(AIModel).filter(
             AIModel.is_enabled == True,
-            func.json_contains(AIModel.capabilities, f'"{capability}"'),
+            AIModel.capabilities.cast(JSONB).contains(type_coerce([capability], JSONB)),
         ).first()
         if m:
             return m