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/repository.py

@@ -2,12 +2,13 @@
 
 import hashlib
 import secrets
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timezone
 
 from sqlalchemy.orm import Session
 
 import duty_teller.config as config
 from duty_teller.db.models import User, Duty, GroupDutyPin, CalendarSubscriptionToken
+from duty_teller.utils.dates import parse_utc_iso_naive, to_date_exclusive_iso
 
 
 def get_user_by_telegram_id(session: Session, telegram_user_id: int) -> User | None:
@@ -168,9 +169,7 @@ def delete_duties_in_range(
     Returns:
         Number of duties deleted.
     """
-    to_next = (
-        datetime.fromisoformat(to_date + "T00:00:00") + timedelta(days=1)
-    ).strftime("%Y-%m-%d")
+    to_next = to_date_exclusive_iso(to_date)
     q = session.query(Duty).filter(
         Duty.user_id == user_id,
         Duty.start_at < to_next,
@@ -197,9 +196,7 @@ def get_duties(
     Returns:
         List of (Duty, full_name) tuples.
     """
-    to_date_next = (
-        datetime.fromisoformat(to_date + "T00:00:00") + timedelta(days=1)
-    ).strftime("%Y-%m-%d")
+    to_date_next = to_date_exclusive_iso(to_date)
     q = (
         session.query(Duty, User.full_name)
         .join(User, Duty.user_id == User.id)
@@ -230,9 +227,7 @@ def get_duties_for_user(
     Returns:
         List of (Duty, full_name) tuples.
     """
-    to_date_next = (
-        datetime.fromisoformat(to_date + "T00:00:00") + timedelta(days=1)
-    ).strftime("%Y-%m-%d")
+    to_date_next = to_date_exclusive_iso(to_date)
     filters = [
         Duty.user_id == user_id,
         Duty.start_at < to_date_next,
@@ -392,9 +387,7 @@ def get_next_shift_end(session: Session, after_utc: datetime) -> datetime | None
         .first()
     )
     if current:
-        return datetime.fromisoformat(current.end_at.replace("Z", "+00:00")).replace(
-            tzinfo=None
-        )
+        return parse_utc_iso_naive(current.end_at)
     next_duty = (
         session.query(Duty)
         .filter(Duty.event_type == "duty", Duty.start_at > after_iso)
@@ -402,9 +395,7 @@ def get_next_shift_end(session: Session, after_utc: datetime) -> datetime | None
         .first()
     )
     if next_duty:
-        return datetime.fromisoformat(next_duty.end_at.replace("Z", "+00:00")).replace(
-            tzinfo=None
-        )
+        return parse_utc_iso_naive(next_duty.end_at)
     return None