feat: add admin user system with role-based access control

Implement comprehensive admin user system for Kubernetes deployment:

Backend:
- Add isAdmin field to User model for role-based permissions
- Create adminAuth middleware to protect admin-only routes
- Protect 11 routes across rewards, cache, streets, and analytics endpoints
- Update setup-couchdb.js to seed default admin user at deployment

Kubernetes:
- Add ADMIN_EMAIL and ADMIN_PASSWORD to secrets.yaml
- Add ADMIN_EMAIL to configmap.yaml for non-sensitive config
- Create couchdb-init-job.yaml for automated database initialization
- Update secrets.yaml.example with admin user documentation

Frontend:
- Create AdminRoute component for admin-only page protection
- Create comprehensive AdminDashboard with 5 tabs:
  * Overview: Platform statistics and quick actions
  * Users: List, search, manage admin status, delete users
  * Streets: Create, edit, delete streets
  * Rewards: Create, edit, toggle, delete rewards
  * Content: Moderate posts and events
- Add Admin navigation link in Navbar (visible only to admins)
- Integrate admin routes in App.js

Default admin user:
- Email: will@wills-portal.com
- Created automatically by K8s init job at deployment

Routes protected:
- POST/PUT/DELETE /api/rewards (catalog management)
- POST /api/streets (street creation)
- DELETE /api/cache (cache operations)
- GET /api/analytics/* (platform statistics)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/Navbar.js

@@ -31,12 +31,19 @@ const Navbar = () => {
       <li className="nav-item">
         <Link className="nav-link" to="/profile" data-testid="nav-profile">Profile</Link>
       </li>
-      <li className="nav-item">
-        <Link className="nav-link" to="/premium" data-testid="nav-premium">Premium</Link>
-      </li>
-      <li className="nav-item">
-        <a onClick={logout} href="#!" className="nav-link" data-testid="logout-button">Logout</a>
-      </li>
+       <li className="nav-item">
+         <Link className="nav-link" to="/premium" data-testid="nav-premium">Premium</Link>
+       </li>
+       {auth.user?.isAdmin && (
+         <li className="nav-item">
+           <Link className="nav-link text-danger" to="/admin" data-testid="nav-admin">
+             Admin
+           </Link>
+         </li>
+       )}
+       <li className="nav-item">
+         <a onClick={logout} href="#!" className="nav-link" data-testid="logout-button">Logout</a>
+       </li>
     </ul>
   );