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.

tests/test_repository_duty_range.py

@@ -9,9 +9,12 @@ from duty_teller.db.repository import (
     delete_duties_in_range,
     get_duties,
     get_duties_for_user,
+    get_duty_by_id,
     get_or_create_user,
     get_or_create_user_by_full_name,
+    get_users_for_admin,
     insert_duty,
+    update_duty_user,
     update_user_display_name,
 )
 
@@ -217,6 +220,52 @@ def test_get_or_create_user_keeps_name_when_flag_true_updates_username(session):
     assert u2.username == "new_username"
 
 
+def test_get_duty_by_id_returns_duty(session, user_a):
+    """get_duty_by_id returns the duty when it exists."""
+    duty = insert_duty(
+        session, user_a.id, "2026-02-01T09:00:00Z", "2026-02-01T18:00:00Z"
+    )
+    found = get_duty_by_id(session, duty.id)
+    assert found is not None
+    assert found.id == duty.id
+    assert found.user_id == user_a.id
+    assert found.start_at == "2026-02-01T09:00:00Z"
+
+
+def test_get_duty_by_id_returns_none_when_missing(session):
+    """get_duty_by_id returns None for non-existent id."""
+    assert get_duty_by_id(session, 99999) is None
+
+
+def test_update_duty_user_changes_user(session, user_a):
+    """update_duty_user updates user_id and returns the duty."""
+    user_b = get_or_create_user_by_full_name(session, "User B")
+    duty = insert_duty(
+        session, user_a.id, "2026-02-01T09:00:00Z", "2026-02-01T18:00:00Z"
+    )
+    updated = update_duty_user(session, duty.id, user_b.id, commit=True)
+    assert updated is not None
+    assert updated.id == duty.id
+    assert updated.user_id == user_b.id
+    session.refresh(duty)
+    assert duty.user_id == user_b.id
+
+
+def test_update_duty_user_returns_none_when_duty_missing(session, user_a):
+    """update_duty_user returns None when duty does not exist."""
+    assert update_duty_user(session, 99999, user_a.id, commit=True) is None
+
+
+def test_get_users_for_admin_returns_all_ordered_by_full_name(session, user_a):
+    """get_users_for_admin returns all users ordered by full_name."""
+    get_or_create_user_by_full_name(session, "Alice")
+    get_or_create_user_by_full_name(session, "Борис")
+    users = get_users_for_admin(session)
+    assert len(users) >= 3
+    full_names = [u.full_name for u in users]
+    assert full_names == sorted(full_names)
+
+
 def test_update_user_display_name_sets_flag_then_get_or_create_user_keeps_name(session):
     """update_user_display_name sets name and flag; get_or_create_user then does not overwrite name."""
     get_or_create_user(