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/services/admin-user.service.ts

@@ -0,0 +1,40 @@
+import { Injectable, inject } from '@angular/core';
+import { HttpClient } from '@angular/common/http';
+import { Observable } from 'rxjs';
+
+import { AdminUser } from '../interfaces/admin-user';
+import { getAppConfig } from './app-config.service';
+
+export interface CreateAdminPayload {
+  email: string;
+  display_name: string;
+  password: string;
+}
+
+export interface UpdateAdminPayload {
+  display_name?: string;
+}
+
+@Injectable({
+  providedIn: 'root',
+})
+export class AdminUserService {
+  private http = inject(HttpClient);
+  private baseUrl = `${getAppConfig().apiBaseUrl}/api/admin/users`;
+
+  getAdminUsers(): Observable<AdminUser[]> {
+    return this.http.get<AdminUser[]>(this.baseUrl);
+  }
+
+  createAdminUser(payload: CreateAdminPayload): Observable<AdminUser> {
+    return this.http.post<AdminUser>(this.baseUrl, payload);
+  }
+
+  updateAdminUser(id: number, payload: UpdateAdminPayload): Observable<AdminUser> {
+    return this.http.put<AdminUser>(`${this.baseUrl}/${id}`, payload);
+  }
+
+  deleteAdminUser(id: number): Observable<void> {
+    return this.http.delete<void>(`${this.baseUrl}/${id}`);
+  }
+}