Add admin user management API and frontend UI

Backend:
- /api/admin/users: GET list, POST create, PUT update, DELETE
- Admin users can be added with email, display name, and password
- Self-deletion and last-admin deletion are blocked
- Admin role auto-assigned on creation

Frontend:
- AdminUsersTabComponent: list + add/delete admin users
- AdminUserService: HTTP client for admin user CRUD
- AdminUser interface
- 'Admins' tab added to admin dashboard

Tests:
- 10 new tests for admin user management endpoints
- 140 total tests pass (130 original + 10 new)

src/app/admin/admin.component.ts

@@ -26,6 +26,7 @@ import { AdminTeamFormComponent } from './admin-team-form.component';
 import { AdminCommunityFormComponent } from './admin-community-form.component';
 import { AdminUnderwriterFormComponent } from './admin-underwriter-form.component';
 import { AdminStatsDashboardComponent } from './admin-stats-dashboard.component';
+import { AdminUsersTabComponent } from './admin-users-tab.component';
 
 type TabKey =
   | 'shows'
@@ -37,7 +38,8 @@ type TabKey =
   | 'theme'
   | 'underwriters'
   | 'mobile-builds'
-  | 'stats';
+  | 'stats'
+  | 'admins';
 
 @Component({
   selector: 'app-admin',
@@ -55,6 +57,7 @@ type TabKey =
     AdminUnderwritersTabComponent,
     AdminMobileBuildsTabComponent,
     AdminStatsDashboardComponent,
+    AdminUsersTabComponent,
     AdminShowFormComponent,
     AdminEventFormComponent,
     AdminStationFormComponent,