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/config.py

@@ -15,7 +15,12 @@ if not (Path("/.dockerenv").exists() or Path("/run/.containerenv").exists()):
         load_dotenv(env_path, override=False)
 
 # 数据库配置
-DATABASE_URL = f"mysql+pymysql://{os.getenv('MYSQL_USER', 'root')}:{os.getenv('MYSQL_PASSWORD', 'pwd2026')}@{os.getenv('MYSQL_HOST', 'localhost')}:{os.getenv('MYSQL_PORT', '3306')}/{os.getenv('MYSQL_DATABASE', 'aiflow')}?charset=utf8mb4"
+# PostgreSQL preferred when DATABASE_URL is provided (production); else legacy MySQL builder (local/dev + rollback)
+_DATABASE_URL_ENV = os.getenv("DATABASE_URL", "")
+if _DATABASE_URL_ENV:
+    DATABASE_URL = _DATABASE_URL_ENV
+else:
+    DATABASE_URL = f"mysql+pymysql://{os.getenv('MYSQL_USER', 'root')}:{os.getenv('MYSQL_PASSWORD', 'pwd2026')}@{os.getenv('MYSQL_HOST', 'localhost')}:{os.getenv('MYSQL_PORT', '3306')}/{os.getenv('MYSQL_DATABASE', 'aiflow')}?charset=utf8mb4"
 
 # Redis配置
 REDIS_HOST = os.getenv("REDIS_HOST", "localhost")