fix: use standard logging format to avoid reserved 'message' key

Python's logging.LogRecord reserves 'message' as an attribute.
Using it in extra dict causes KeyError.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/dependencies/auth.py

@@ -131,11 +131,8 @@ async def get_current_agent_compat(
     # Fall back to legacy Bearer token authentication
     if authorization:
         logger.warning(
-            "deprecated_auth_scheme",
-            extra={
-                "message": "Bearer token auth is deprecated. Use X-Agent-Id and X-Agent-Secret headers.",
-                "agent_id": str(agent_id) if agent_id else None,
-            },
+            "deprecated_auth_scheme: Bearer token auth is deprecated. Use X-Agent-Id and X-Agent-Secret headers. agent_id=%s",
+            str(agent_id) if agent_id else None,
         )
 
         # Parse Bearer token