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/core/observability.py

@@ -6,7 +6,7 @@ Qihu-AIFlow 统一可观测基座 (Phase 2.1 + Phase 1.I OTel 增强)
 
 设计原则（Strangler Fig 增量）：
 - 纯增量埋点，不改任何既有接口的返回结构。
-- 向后兼容现有 `logging.getLogger(__name__)` 用法：trace_id 由 root logger 的
+- 向后兼容现有 \"logging.getLogger(__name__)\" 用法：trace_id 由 root logger 的
   TraceIdFilter 自动注入，调用方无需改动。
 - 不引入第三方依赖（structlog 未安装），自带可选的 JSON formatter。
 
@@ -58,7 +58,7 @@ def get_trace_id() -> str:
 class TraceIdFilter(logging.Filter):
     """为每一条日志记录注入 trace_id 属性（不存在则写为 '-'）。
 
-    挂载到 root logger 后，所有 `getLogger(__name__)` 的子 logger 都会继承，
+    挂载到 root logger 后，所有 \"getLogger(__name__)\" 的子 logger 都会继承，
     从而让全平台既有日志自动带上链路 ID，零侵入。
     """