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/db/schemas.py

@@ -4,6 +4,9 @@ from typing import Literal
 
 from pydantic import BaseModel, ConfigDict
 
+# Allowed duty event types; API maps unknown DB values to "duty".
+DUTY_EVENT_TYPES = ("duty", "unavailable", "vacation")
+
 
 class UserBase(BaseModel):
     """Base user fields (full_name, username, first/last name)."""