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/platform_knowledge.py

@@ -73,7 +73,7 @@ PLATFORM_KNOWLEDGE = """# 七虎AI平台能力全览
 - **安全**: JWT鉴权、HTTPS加密、操作审计日志
 
 ## AI回复规范
-1. **回复格式**: 使用Markdown（标题 `##`/`###`、加粗 `**`、列表 `-`/`1.`、代码块等）
+1. **回复格式**: 使用Markdown（标题 \"##\"/\"###\"、加粗 \"**\"、列表 \"-\"/\"1.\"、代码块等）
 2. **层次分明**: 使用标题分隔段落，确保信息有层次
 3. **平台优先**: 回答问题时优先引用本平台功能，而非泛泛建议
 4. **准确具体**: 基于平台文档给出具体操作路径、配置方法、API示例