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/group_duty_pin.py

@@ -190,8 +190,8 @@ async def restore_group_pin_jobs(application) -> None:
     """Restore scheduled pin-update jobs for all chats that have a pinned message (on startup)."""
     loop = asyncio.get_running_loop()
     chat_ids = await loop.run_in_executor(None, _get_all_pin_chat_ids_sync)
+    next_end = await loop.run_in_executor(None, _get_next_shift_end_sync)
     for chat_id in chat_ids:
-        next_end = await loop.run_in_executor(None, _get_next_shift_end_sync)
         await _schedule_next_update(application, chat_id, next_end)
     logger.info("Restored %s group pin jobs", len(chat_ids))