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-lock.json

@@ -15,6 +15,7 @@
         "cors": "^2.8.5",
         "dotenv": "^16.4.7",
         "express": "^4.21.2",
+        "express-mongo-sanitize": "^2.2.0",
         "express-rate-limit": "^8.2.1",
         "express-validator": "^7.3.0",
         "globals": "^16.4.0",
@@ -22,8 +23,10 @@
         "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"
+        "stripe": "^17.7.0",
+        "xss-clean": "^0.1.4"
       },
       "devDependencies": {
         "@types/jest": "^30.0.0",
@@ -2055,6 +2058,15 @@
         "url": "https://github.com/chalk/wrap-ansi?sponsor=1"
       }
     },
+    "node_modules/clone": {
+      "version": "2.1.2",
+      "resolved": "https://registry.npmjs.org/clone/-/clone-2.1.2.tgz",
+      "integrity": "sha512-3Pe/CF1Nn94hyhIYpjtiLhdCoEoz0DqQ+988E9gmeEdQZlojxnOb74wctFyuwWQHzqyf9X7C7MG8juUpqBJT8w==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.8"
+      }
+    },
     "node_modules/cloudinary": {
       "version": "2.8.0",
       "license": "MIT",
@@ -2755,6 +2767,15 @@
         "url": "https://opencollective.com/express"
       }
     },
+    "node_modules/express-mongo-sanitize": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/express-mongo-sanitize/-/express-mongo-sanitize-2.2.0.tgz",
+      "integrity": "sha512-PZBs5nwhD6ek9ZuP+W2xmpvcrHwXZxD5GdieX2dsjPbAbH4azOkrHbycBud2QRU+YQF1CT+pki/lZGedHgo/dQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=10"
+      }
+    },
     "node_modules/express-rate-limit": {
       "version": "8.2.1",
       "license": "MIT",
@@ -4396,6 +4417,18 @@
       "version": "3.1.1",
       "license": "MIT"
     },
+    "node_modules/node-cache": {
+      "version": "5.1.2",
+      "resolved": "https://registry.npmjs.org/node-cache/-/node-cache-5.1.2.tgz",
+      "integrity": "sha512-t1QzWwnk4sjLWaQAS8CHgOJ+RAfmHpxFWmc36IWTiWHQfs0w5JDMBS1b1ZxQteo0vVVuWJvIUKHDkkeK7vIGCg==",
+      "license": "MIT",
+      "dependencies": {
+        "clone": "2.x"
+      },
+      "engines": {
+        "node": ">= 8.0.0"
+      }
+    },
     "node_modules/node-int64": {
       "version": "0.4.0",
       "dev": true,
@@ -5852,6 +5885,21 @@
         "node": ">=0.4.0"
       }
     },
+    "node_modules/xss-clean": {
+      "version": "0.1.4",
+      "resolved": "https://registry.npmjs.org/xss-clean/-/xss-clean-0.1.4.tgz",
+      "integrity": "sha512-4hArTFHYxrifK9VXOu/zFvwjTXVjKByPi6woUHb1IqxlX0Z4xtFBRjOhTKuYV/uE1VswbYsIh5vUEYp7MmoISQ==",
+      "deprecated": "Package no longer supported. Contact Support at https://www.npmjs.com/support for more info.",
+      "license": "MIT",
+      "dependencies": {
+        "xss-filters": "1.2.7"
+      }
+    },
+    "node_modules/xss-filters": {
+      "version": "1.2.7",
+      "resolved": "https://registry.npmjs.org/xss-filters/-/xss-filters-1.2.7.tgz",
+      "integrity": "sha512-KzcmYT/f+YzcYrYRqw6mXxd25BEZCxBQnf+uXTopQDIhrmiaLwO+f+yLsIvvNlPhYvgff8g3igqrBxYh9k8NbQ=="
+    },
     "node_modules/xtend": {
       "version": "4.0.2",
       "license": "MIT",