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/models.py

@@ -14,8 +14,8 @@ class User(Base):
     __tablename__ = "users"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
-    telegram_user_id: Mapped[int] = mapped_column(
-        BigInteger, unique=True, nullable=False
+    telegram_user_id: Mapped[int | None] = mapped_column(
+        BigInteger, unique=True, nullable=True
     )
     full_name: Mapped[str] = mapped_column(Text, nullable=False)
     username: Mapped[str | None] = mapped_column(Text, nullable=True)