refactor(observability): Phase2.2 成本归因 trace_id列+token日志写入+LLM/MCP/媒体子span [v2026.07.19.3]

backend/python/app/models.py

@@ -537,6 +537,7 @@ class TokenUsageLog(Base):
     app_id = Column(String(36), default=None)
     app_name = Column(String(200), default=None)
     conversation_id = Column(String(36), default=None)
+    trace_id = Column(String(64), default=None)
     prompt_tokens = Column(Integer, default=0)
     completion_tokens = Column(Integer, default=0)
     total_tokens = Column(Integer, default=0)