feat: implement API response caching with node-cache

- Add in-memory cache middleware with configurable TTL
- Cache GET endpoints: streets (5min), events (2min), posts (1min), rewards (10min)
- Automatic cache invalidation on POST/PUT/DELETE operations
- Add cache statistics endpoint (GET /api/cache/stats)
- Add cache management endpoint (DELETE /api/cache)
- Cache hit rate tracking and monitoring
- Pattern-based cache invalidation
- Optimized for Raspberry Pi deployment (lightweight in-memory)

🤖 Generated with Claude

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

backend/routes/rewards.js

@@ -7,6 +7,7 @@ const {
   rewardIdValidation,
 } = require("../middleware/validators/rewardValidator");
 const { paginate, buildPaginatedResponse } = require("../middleware/pagination");
+const { getCacheMiddleware, invalidateCacheByPattern } = require("../middleware/cache");
 
 const router = express.Router();
 
@@ -14,6 +15,7 @@ const router = express.Router();
 router.get(
   "/",
   paginate,
+  getCacheMiddleware(600), // Cache for 10 minutes
   asyncHandler(async (req, res) => {
     const { page, limit } = req.pagination;
 
@@ -37,6 +39,9 @@ router.post(
       isPremium,
     });
 
+    // Invalidate rewards cache
+    invalidateCacheByPattern('/api/rewards');
+
     res.json(reward);
   }),
 );