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/commands.py

@@ -11,6 +11,7 @@ from duty_teller.db.repository import (
     get_or_create_user,
     set_user_phone,
     create_calendar_token,
+    is_admin_for_telegram_user,
 )
 from duty_teller.i18n import get_lang, t
 from duty_teller.utils.user import build_full_name
@@ -150,7 +151,13 @@ async def help_cmd(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
         t(lang, "help.calendar_link"),
         t(lang, "help.pin_duty"),
     ]
-    if config.is_admin(update.effective_user.username or ""):
+
+    def check_admin() -> bool:
+        with session_scope(config.DATABASE_URL) as session:
+            return is_admin_for_telegram_user(session, update.effective_user.id)
+
+    is_admin_user = await asyncio.get_running_loop().run_in_executor(None, check_admin)
+    if is_admin_user:
         lines.append(t(lang, "help.import_schedule"))
     await update.message.reply_text("\n".join(lines))