feat: implement admin panel functionality in Mini App

- Added new API endpoints for admin features: `GET /api/admin/me`, `GET /api/admin/users`, and `PATCH /api/admin/duties/:id` to manage user duties.
- Introduced `UserForAdmin` and `AdminDutyReassignBody` schemas for handling admin-related data.
- Updated documentation to include Mini App design guidelines and admin panel functionalities.
- Enhanced tests for admin API to ensure proper access control and functionality.
- Improved error handling and localization for admin actions.

duty_teller/db/repository.py

@@ -322,6 +322,61 @@ def delete_duties_in_range(
     return count
 
 
+def get_duty_by_id(session: Session, duty_id: int) -> Duty | None:
+    """Return duty by primary key.
+
+    Args:
+        session: DB session.
+        duty_id: Duty id (duties.id).
+
+    Returns:
+        Duty or None if not found.
+    """
+    return session.get(Duty, duty_id)
+
+
+def update_duty_user(
+    session: Session,
+    duty_id: int,
+    new_user_id: int,
+    *,
+    commit: bool = True,
+) -> Duty | None:
+    """Update the assigned user of a duty.
+
+    Args:
+        session: DB session.
+        duty_id: Duty id (duties.id).
+        new_user_id: New user id (users.id).
+        commit: If True, commit immediately. If False, caller commits.
+
+    Returns:
+        Updated Duty or None if duty not found.
+    """
+    duty = session.get(Duty, duty_id)
+    if duty is None:
+        return None
+    duty.user_id = new_user_id
+    if commit:
+        session.commit()
+        session.refresh(duty)
+    else:
+        session.flush()
+    return duty
+
+
+def get_users_for_admin(session: Session) -> list[User]:
+    """Return all users ordered by full_name for admin dropdown (id, full_name, username).
+
+    Args:
+        session: DB session.
+
+    Returns:
+        List of User instances ordered by full_name.
+    """
+    return session.query(User).order_by(User.full_name).all()
+
+
 def get_duties(
     session: Session,
     from_date: str,