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/admin/billing_admin.py

@@ -445,7 +445,7 @@ async def billing_stats(db: Session = Depends(get_db), user=Depends(get_current_
         SELECT DATE(created_at) as d, COALESCE(SUM(amount), 0) as rev
         FROM billing_transactions
         WHERE transaction_type='recharge' AND status='completed'
-          AND created_at >= DATE_SUB(NOW(), INTERVAL 7 DAY)
+          AND created_at >= NOW() - INTERVAL '7 DAY'
         GROUP BY DATE(created_at) ORDER BY d
     """)).fetchall()
     daily_revenue = [{"date": str(r[0]), "revenue": float(r[1])} for r in revenue_7d]