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/middleware/cache.js

@@ -0,0 +1,100 @@
+const NodeCache = require('node-cache');
+const logger = require('../utils/logger');
+
+// Initialize cache with default TTL of 5 minutes and check period of 2 minutes
+const cache = new NodeCache({ stdTTL: 300, checkperiod: 120 });
+
+/**
+ * Cache middleware factory
+ * @param {number} ttlSeconds - Time to live in seconds for cached responses
+ * @returns {Function} Express middleware function
+ */
+const getCacheMiddleware = (ttlSeconds) => (req, res, next) => {
+  const key = req.originalUrl;
+  const cachedResponse = cache.get(key);
+
+  if (cachedResponse) {
+    logger.info(`Cache hit for key: ${key}`);
+    return res.json(cachedResponse);
+  }
+
+  logger.info(`Cache miss for key: ${key}`);
+  
+  // Store the original res.json method
+  const originalJson = res.json.bind(res);
+  
+  // Override res.json to cache the response
+  res.json = (body) => {
+    cache.set(key, body, ttlSeconds);
+    return originalJson(body);
+  };
+  
+  next();
+};
+
+/**
+ * Invalidate cache by exact key(s)
+ * @param {string|string[]} keys - Cache key or array of keys to invalidate
+ */
+const invalidateCache = (keys) => {
+  if (Array.isArray(keys)) {
+    keys.forEach(key => {
+      logger.info(`Invalidating cache for key: ${key}`);
+      cache.del(key);
+    });
+  } else {
+    logger.info(`Invalidating cache for key: ${keys}`);
+    cache.del(keys);
+  }
+};
+
+/**
+ * Invalidate all cache keys matching a pattern
+ * @param {string} pattern - Pattern to match (keys starting with this string)
+ */
+const invalidateCacheByPattern = (pattern) => {
+  const keys = cache.keys();
+  const keysToDelete = keys.filter(key => key.startsWith(pattern));
+  
+  if (keysToDelete.length > 0) {
+    logger.info(`Invalidating cache by pattern: ${pattern}, deleted ${keysToDelete.length} keys`);
+    cache.del(keysToDelete);
+  } else {
+    logger.debug(`No cache keys found matching pattern: ${pattern}`);
+  }
+};
+
+/**
+ * Get cache statistics
+ * @returns {Object} Cache statistics
+ */
+const getCacheStats = () => {
+  const stats = cache.getStats();
+  const keys = cache.keys();
+  
+  return {
+    keys: keys.length,
+    hits: stats.hits,
+    misses: stats.misses,
+    hitRate: stats.hits > 0 ? (stats.hits / (stats.hits + stats.misses) * 100).toFixed(2) + '%' : '0%',
+    ksize: stats.ksize,
+    vsize: stats.vsize
+  };
+};
+
+/**
+ * Clear all cache
+ */
+const clearCache = () => {
+  const keyCount = cache.keys().length;
+  cache.flushAll();
+  logger.info(`Cleared all cache (${keyCount} keys)`);
+};
+
+module.exports = {
+  getCacheMiddleware,
+  invalidateCache,
+  invalidateCacheByPattern,
+  getCacheStats,
+  clearCache
+};