feat: implement caching for duty-related data and enhance performance

- Added a TTLCache class for in-memory caching of duty-related data, improving performance by reducing database queries.
- Integrated caching into the group duty pin functionality, allowing for efficient retrieval of message text and next shift end times.
- Introduced new methods to invalidate caches when relevant data changes, ensuring data consistency.
- Created a new Alembic migration to add indexes on the duties table for improved query performance.
- Updated tests to cover the new caching behavior and ensure proper functionality.

duty_teller/db/repository.py

@@ -229,6 +229,22 @@ def get_or_create_user_by_full_name(session: Session, full_name: str) -> User:
     return user
 
 
+def get_users_by_full_names(session: Session, full_names: list[str]) -> dict[str, User]:
+    """Get users by full_name. Returns dict full_name -> User. Does not create missing.
+
+    Args:
+        session: DB session.
+        full_names: List of full names to look up.
+
+    Returns:
+        Dict mapping full_name to User for found users.
+    """
+    if not full_names:
+        return {}
+    users = session.query(User).filter(User.full_name.in_(full_names)).all()
+    return {u.full_name: u for u in users}
+
+
 def update_user_display_name(
     session: Session,
     telegram_user_id: int,
@@ -268,6 +284,8 @@ def delete_duties_in_range(
     user_id: int,
     from_date: str,
     to_date: str,
+    *,
+    commit: bool = True,
 ) -> int:
     """Delete all duties of the user that overlap the given date range.
 
@@ -276,6 +294,7 @@ def delete_duties_in_range(
         user_id: User id.
         from_date: Start date YYYY-MM-DD.
         to_date: End date YYYY-MM-DD.
+        commit: If True, commit immediately. If False, caller commits (for batch import).
 
     Returns:
         Number of duties deleted.
@@ -288,7 +307,8 @@ def delete_duties_in_range(
     )
     count = q.count()
     q.delete(synchronize_session=False)
-    session.commit()
+    if commit:
+        session.commit()
     return count