TUI von direkten Service-Importen auf httpx API-Calls umgestellt.
Neue tui/api_client.py als schlanker async HTTP-Client. Background-
Worker pollt API alle 5s für Echtzeit-Updates. Neuer tui_standalone.py
Entry-Point für 'textual serve --port 8001 tui_standalone.py'.
MultiCustomerMessenger supporting Telegram (python-telegram-bot),
WhatsApp (Green API) and SMS (python-gsmmodem-new). REST API with
Bearer-token auth, SQLAlchemy models for MariaDB, APScheduler for
background polling, and Textual TUI running in same asyncio event-loop.
