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/server.js

@@ -136,6 +136,7 @@ const badgesRoutes = require("./routes/badges");
 const aiRoutes = require("./routes/ai");
 const paymentRoutes = require("./routes/payments");
 const userRoutes = require("./routes/users");
+const cacheRoutes = require("./routes/cache");
 
 // Apply rate limiters
 app.use("/api/auth/register", authLimiter);
@@ -236,6 +237,7 @@ app.use("/api/badges", badgesRoutes);
 app.use("/api/ai", aiRoutes);
 app.use("/api/payments", paymentRoutes);
 app.use("/api/users", userRoutes);
+app.use("/api/cache", cacheRoutes);
 
 app.get("/", (req, res) => {
   res.send("Street Adoption App Backend");