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/im_bot_platform.py

@@ -109,20 +109,20 @@ def _ensure_table(db):
     """确保 im_bot_platform_account 表存在（首次调用时创建）。"""
     db.execute(text("""
         CREATE TABLE IF NOT EXISTS im_bot_platform_account (
-            id INT AUTO_INCREMENT PRIMARY KEY,
+            id SERIAL PRIMARY KEY,
             user_id VARCHAR(64) DEFAULT NULL,
             platform VARCHAR(32) NOT NULL,
             status VARCHAR(32) DEFAULT 'disconnected',
-            creds MEDIUMTEXT NULL,
+            creds TEXT NULL,
             send_url VARCHAR(512) NULL,
             api_base VARCHAR(512) NULL,
             bot_token VARCHAR(512) NULL,
             webhook_token VARCHAR(64) NULL,
-            last_error MEDIUMTEXT NULL,
-            created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
-            updated_at DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
-            UNIQUE KEY uk_user_platform (user_id, platform)
-        ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
+            last_error TEXT NULL,
+            created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+            updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+            UNIQUE (user_id, platform)
+        )
     """))
     db.commit()