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/chat_media.py

@@ -3,7 +3,8 @@
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Response
 from sqlalchemy.orm import Session
-from sqlalchemy import and_, desc, asc, func
+from sqlalchemy import and_, desc, asc, func, type_coerce
+from sqlalchemy.dialects.postgresql import JSONB
 from pydantic import BaseModel, Field
 from typing import Optional
 import uuid
@@ -893,7 +894,7 @@ def _build_media_route(db, capability: str, preferred_id: str = None):
 
     others = db.query(_AM).filter(
         _AM.is_enabled == True,
-        func.json_contains(_AM.capabilities, f'"{capability}"'),
+        _AM.capabilities.cast(JSONB).contains(type_coerce([capability], JSONB)),
     ).order_by(_AM.is_default.desc(), _AM.created_at.asc()).all()
     for m in others:
         _add(m)