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/common.py

@@ -0,0 +1,24 @@
+"""Shared handler helpers (e.g. async admin check)."""
+
+import asyncio
+
+import duty_teller.config as config
+from duty_teller.db.repository import is_admin_for_telegram_user
+from duty_teller.db.session import session_scope
+
+
+async def is_admin_async(telegram_user_id: int) -> bool:
+    """Check if Telegram user is admin (username or phone). Runs DB check in executor.
+
+    Args:
+        telegram_user_id: Telegram user id.
+
+    Returns:
+        True if user is in ADMIN_USERNAMES or their stored phone is in ADMIN_PHONES.
+    """
+
+    def _check() -> bool:
+        with session_scope(config.DATABASE_URL) as session:
+            return is_admin_for_telegram_user(session, telegram_user_id)
+
+    return await asyncio.get_running_loop().run_in_executor(None, _check)