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.

docs/architecture.md

@@ -18,6 +18,9 @@ High-level architecture of Duty Teller: components, data flow, and package relat
 - **Miniapp → API**  
   Browser opens `/app`; frontend calls `GET /api/duties` and `GET /api/calendar-events` with date range. FastAPI dependencies: DB session, Telegram initData validation (`require_miniapp_username`), date validation. Data is read via `duty_teller.db.repository`.
 
+- **Admin panel (Mini App)**  
+  Admins see an "Admin" link on the calendar (when `GET /api/admin/me` returns `is_admin: true`). The admin page at `/app/admin` lists duties for the current month and allows reassigning a duty to another user. It uses `GET /api/admin/users` (admin-only) for the user dropdown and `PATCH /api/admin/duties/:id` with `{ user_id }` to reassign. All admin endpoints require valid initData; `/users` and PATCH `/duties` additionally require the user to have the admin role (`require_admin_telegram_id`). PATCH error messages (e.g. duty not found, user not found) use the request `Accept-Language` header for i18n. The reassign dropdown shows only users with role `user` or `admin` (role_id 1 or 2 per migration 007).
+
 - **Import**  
   Admin sends JSON file via `/import_duty_schedule`. Handler reads file → `duty_teller.importers.duty_schedule.parse_duty_schedule()` → `DutyScheduleResult` → `duty_teller.services.import_service.run_import()` → repository (`get_or_create_user_by_full_name`, `delete_duties_in_range`, `insert_duty`).