feat: implement analytics endpoints (task 28)

- Created analytics service with summary, trends, comparison, and aggregated data methods
- Implemented 5 analytics endpoints: summary, trends, comparison, aggregated, and CSV export
- Added time series data with day/week/month interval support
- Implemented period-over-period comparison metrics with percentage changes
- Added CSV export functionality using json2csv
- Updated frontend analytics service to unwrap API responses
- Fixed chart components to work with new data structure (reportCount, eventCount, litterCollected)
- Updated TypeScript types to match backend API structure
- All endpoints tested and verified working

Requirements satisfied: 5.1, 5.2, 5.3, 5.4, 5.5

server/package.json

@@ -29,6 +29,7 @@
     "express": "^4.21.1",
     "express-validator": "^7.2.0",
     "helmet": "^8.0.0",
+    "json2csv": "^6.0.0-alpha.2",
     "multer": "^1.4.5-lts.1",
     "node-cron": "^4.2.1",
     "redis": "^5.9.0",
@@ -37,6 +38,7 @@
   "devDependencies": {
     "@types/cors": "^2.8.17",
     "@types/express": "^5.0.0",
+    "@types/json2csv": "^5.0.7",
     "@types/multer": "^1.4.12",
     "@types/node": "^22.9.0",
     "@types/node-cron": "^3.0.11",