Implement duty schedule import functionality and enhance user management

- Added a new command `/import_duty_schedule` for importing duty schedules via JSON, restricted to admin users.
- Introduced a two-step import process: specifying handover time and uploading a JSON file.
- Updated the database schema to allow `telegram_user_id` to be nullable for user creation by full name.
- Implemented repository functions for user management, including `get_or_create_user_by_full_name` and `delete_duties_in_range`.
- Enhanced README documentation with details on the new import command and JSON format requirements.
- Added comprehensive tests for the duty schedule parser and integration tests for the import functionality.

db/repository.py

@@ -1,5 +1,7 @@
 """Repository: get_or_create_user, get_duties, insert_duty."""
 
+from datetime import datetime, timedelta
+
 from sqlalchemy.orm import Session
 
 from db.models import User, Duty
@@ -35,6 +37,47 @@ def get_or_create_user(
     return user
 
 
+def get_or_create_user_by_full_name(session: Session, full_name: str) -> User:
+    """Find user by exact full_name or create one with telegram_user_id=None (for duty-schedule import)."""
+    user = session.query(User).filter(User.full_name == full_name).first()
+    if user:
+        return user
+    user = User(
+        telegram_user_id=None,
+        full_name=full_name,
+        username=None,
+        first_name=None,
+        last_name=None,
+    )
+    session.add(user)
+    session.commit()
+    session.refresh(user)
+    return user
+
+
+def delete_duties_in_range(
+    session: Session,
+    user_id: int,
+    from_date: str,
+    to_date: str,
+) -> int:
+    """Delete all duties of the user that overlap [from_date, to_date] (YYYY-MM-DD). Returns count deleted."""
+    # start_at < to_date + 1 day so duties starting on to_date are included (start_at is ISO with T)
+    to_next = (datetime.fromisoformat(to_date + "T00:00:00") + timedelta(days=1)).strftime("%Y-%m-%d")
+    q = (
+        session.query(Duty)
+        .filter(
+            Duty.user_id == user_id,
+            Duty.start_at < to_next,
+            Duty.end_at >= from_date,
+        )
+    )
+    count = q.count()
+    q.delete(synchronize_session=False)
+    session.commit()
+    return count
+
+
 def get_duties(
     session: Session,
     from_date: str,