refactor: improve language normalization and date handling utilities

- Introduced a new `normalize_lang` function to standardize language codes across the application, ensuring consistent handling of user language preferences.
- Refactored date handling utilities by adding `parse_utc_iso` and `parse_utc_iso_naive` functions for better parsing of ISO 8601 date strings, enhancing timezone awareness.
- Updated various modules to utilize the new language normalization and date parsing functions, improving code clarity and maintainability.
- Enhanced error handling in date validation to raise specific `DateRangeValidationError` exceptions, providing clearer feedback on validation issues.
- Improved test coverage for date range validation and language normalization functionalities, ensuring robustness and reliability.

duty_teller/handlers/commands.py

@@ -11,8 +11,8 @@ from duty_teller.db.repository import (
     get_or_create_user,
     set_user_phone,
     create_calendar_token,
-    is_admin_for_telegram_user,
 )
+from duty_teller.handlers.common import is_admin_async
 from duty_teller.i18n import get_lang, t
 from duty_teller.utils.user import build_full_name
 
@@ -151,13 +151,7 @@ async def help_cmd(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
         t(lang, "help.calendar_link"),
         t(lang, "help.pin_duty"),
     ]
-
-    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:
+    if await is_admin_async(update.effective_user.id):
         lines.append(t(lang, "help.import_schedule"))
     await update.message.reply_text("\n".join(lines))