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/__init__.py

@@ -3,7 +3,13 @@
 from db.models import Base, User, Duty
 from db.schemas import UserCreate, UserInDb, DutyCreate, DutyInDb, DutyWithUser
 from db.session import get_engine, get_session_factory, get_session
-from db.repository import get_or_create_user, get_duties, insert_duty
+from db.repository import (
+    delete_duties_in_range,
+    get_or_create_user,
+    get_or_create_user_by_full_name,
+    get_duties,
+    insert_duty,
+)
 
 __all__ = [
     "Base",
@@ -17,7 +23,9 @@ __all__ = [
     "get_engine",
     "get_session_factory",
     "get_session",
+    "delete_duties_in_range",
     "get_or_create_user",
+    "get_or_create_user_by_full_name",
     "get_duties",
     "insert_duty",
     "init_db",