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/i18n/lang.py

@@ -0,0 +1,26 @@
+"""Single source of truth for normalizing language codes to 'ru' or 'en'.
+
+Use for: env DEFAULT_LANGUAGE, Accept-Language header, Telegram user.language_code,
+and initData user object in Miniapp auth.
+"""
+
+from typing import Literal
+
+
+def normalize_lang(code: str | None) -> Literal["ru", "en"]:
+    """Normalize a language code to 'ru' or 'en'.
+
+    Suitable for: env DEFAULT_LANGUAGE, Accept-Language header,
+    Telegram user.language_code, and initData user in Miniapp auth.
+
+    Args:
+        code: Raw language code (e.g. 'ru', 'ru-RU', 'en', 'en-US') or None.
+
+    Returns:
+        'ru' if code starts with 'ru' (after strip/lower), else 'en'.
+        Returns 'en' when code is empty or not a string.
+    """
+    if not code or not isinstance(code, str):
+        return "en"
+    code = code.strip().lower()
+    return "ru" if code.startswith("ru") else "en"