fix: Add CancelledError handling for graceful uvicorn shutdown

Handle asyncio.CancelledError during uvicorn shutdown gracefully
instead of treating it as a fatal error. This prevents spurious
error logging when the server receives shutdown signals.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -2313,6 +2313,10 @@ if __name__ == "__main__":
             _server_started = True
             signal.alarm(0)  # Cancel timeout
             log_startup("Uvicorn server stopped normally")
+        except asyncio.CancelledError:
+            signal.alarm(0)  # Cancel timeout
+            log_startup("Server shutdown cancelled gracefully")
+            print("[INFO] Server shutdown requested - exiting cleanly")
         except Exception as launch_error:
             signal.alarm(0)  # Cancel timeout
             # Write to crash.log immediately