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/schemas.py

@@ -69,6 +69,21 @@ class DutyWithUser(DutyInDb):
     model_config = ConfigDict(from_attributes=True)
 
 
+class UserForAdmin(BaseModel):
+    """User summary for admin dropdown: id, full_name, username, role_id."""
+
+    id: int
+    full_name: str
+    username: str | None = None
+    role_id: int | None = None
+
+
+class AdminDutyReassignBody(BaseModel):
+    """Request body for PATCH /api/admin/duties/:id — reassign duty to another user."""
+
+    user_id: int
+
+
 class CalendarEvent(BaseModel):
     """External calendar event (e.g. holiday) for a single day."""