chore: temporarily use logger.warning for on_chain_start observation

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/chat.py

@@ -146,7 +146,7 @@ async def _stream_response(request: ChatRequest) -> AsyncIterator[bytes]:
                 chain_name = event.get("name", "")
                 metadata = event.get("metadata", {})
                 tags = event.get("tags", [])
-                logger.debug(
+                logger.warning(
                     "on_chain_start: name=%s, langgraph_node=%s, tags=%s",
                     chain_name,
                     metadata.get("langgraph_node", ""),