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.

alembic/versions/002_users_telegram_user_id_nullable.py

@@ -0,0 +1,35 @@
+"""Users: telegram_user_id nullable (for import by full_name)
+
+Revision ID: 002
+Revises: 001
+Create Date: 2025-02-17
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "002"
+down_revision: Union[str, None] = "001"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("users", schema=None) as batch_op:
+        batch_op.alter_column(
+            "telegram_user_id",
+            existing_type=sa.BigInteger(),
+            nullable=True,
+        )
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("users", schema=None) as batch_op:
+        batch_op.alter_column(
+            "telegram_user_id",
+            existing_type=sa.BigInteger(),
+            nullable=False,
+        )