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/models.py

@@ -11,7 +11,7 @@ from sqlalchemy import (
     DECIMAL, UniqueConstraint
 )
 from sqlalchemy.orm import relationship
-from sqlalchemy.dialects.mysql import MEDIUMTEXT
+from sqlalchemy import Text  # PG-compatible (was MEDIUMTEXT from mysql dialect)
 from .database import Base
 
 
@@ -300,7 +300,7 @@ class MarketApp(Base):
     rating_avg = Column(DECIMAL(3, 2), default=0.00)
     review_count = Column(Integer, default=0)
     install_count = Column(Integer, default=0)
-    skill_content = Column(MEDIUMTEXT, default=None)
+    skill_content = Column(Text, default=None)
     mcp_transport = Column(String(20), default=None)
     platform = Column(String(20), default=None, comment="Agent 接入平台：dify/n8n/coze")
     capability = Column(String(20), default=None, comment="能力分类：text/thinking/vision/tool/image/audio/asr/ocr/embedding/rerank/video")