Replace the try/except UTC shim in mqtt_handler.py with timezone.utc
directly, consistent with the api_endpoints.py fix. Add a static AST
scan that fails if datetime.UTC is reintroduced anywhere in the codebase,
guarding against future regressions on Python 3.10 (LuckFox Pico Ultra).
Co-Authored-By: Zindello <josh@zindello.com.au>
Zindello