feat: implement role-based access control for miniapp

- Introduced a new roles table in the database to manage user roles ('user' and 'admin') for access control.
- Updated the user model to include a foreign key reference to the roles table, allowing for role assignment.
- Enhanced command handlers to support the `/set_role` command for admins to assign roles to users.
- Refactored access control logic to utilize role checks instead of username/phone allowlists, improving security and maintainability.
- Updated documentation to reflect changes in access control mechanisms and role management.
- Added unit tests to ensure correct functionality of role assignment and access checks.

duty_teller/db/__init__.py

@@ -1,6 +1,6 @@
 """Database layer: SQLAlchemy models, Pydantic schemas, repository, init."""
 
-from duty_teller.db.models import Base, User, Duty
+from duty_teller.db.models import Base, User, Duty, Role
 from duty_teller.db.schemas import (
     UserCreate,
     UserInDb,
@@ -28,6 +28,7 @@ __all__ = [
     "Base",
     "User",
     "Duty",
+    "Role",
     "UserCreate",
     "UserInDb",
     "DutyCreate",