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/package.json

@@ -31,6 +31,7 @@
     "jsonwebtoken": "^9.0.2",
     "multer": "^1.4.5-lts.1",
     "nano": "^10.1.4",
+    "node-cache": "^5.1.2",
     "socket.io": "^4.8.1",
     "stripe": "^17.7.0",
     "xss-clean": "^0.1.4"