refactor: streamline configuration loading and enhance admin checks

- Refactored the configuration loading in `config.py` to utilize a single source of truth through the `Settings` class, improving maintainability and clarity.
- Introduced the `is_admin_for_telegram_user` function in `repository.py` to centralize admin checks based on usernames and phone numbers.
- Updated command handlers to use the new admin check function, ensuring consistent access control across the application.
- Enhanced error handling in the `error_handler` to log exceptions when sending error replies to users, improving debugging capabilities.
- Improved the handling of user phone updates in `repository.py` to ensure proper normalization and validation of phone numbers.

duty_teller/handlers/errors.py

@@ -22,6 +22,9 @@ async def error_handler(
     """
     logger.exception("Exception while handling an update")
     if isinstance(update, Update) and update.effective_message:
-        user = getattr(update, "effective_user", None)
-        lang = get_lang(user) if user else config.DEFAULT_LANGUAGE
-        await update.effective_message.reply_text(t(lang, "errors.generic"))
+        try:
+            user = getattr(update, "effective_user", None)
+            lang = get_lang(user) if user else config.DEFAULT_LANGUAGE
+            await update.effective_message.reply_text(t(lang, "errors.generic"))
+        except Exception:
+            logger.warning("Could not send error reply to user", exc_info=True)