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>

backend/routes/cache.js

@@ -1,5 +1,6 @@
 const express = require("express");
 const auth = require("../middleware/auth");
+const adminAuth = require("../middleware/adminAuth");
 const { asyncHandler } = require("../middleware/errorHandler");
 const { getCacheStats, clearCache } = require("../middleware/cache");
 
@@ -31,6 +32,7 @@ router.get(
 router.delete(
   "/",
   auth,
+  adminAuth,
   asyncHandler(async (req, res) => {
     clearCache();
     res.json({