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

src/components/analytics/TimeSeriesChart.tsx

@@ -57,17 +57,17 @@ export function TimeSeriesChart({ data, loading }: TimeSeriesChartProps) {
           />
           <Line 
             type="monotone" 
-            dataKey="reports" 
+            dataKey="reportCount" 
             stroke="#3b82f6" 
             strokeWidth={2}
             name="Reports"
             dot={{ fill: '#3b82f6', r: 4 }}
             activeDot={{ r: 6 }}
           />
-          {data.some(d => d.events !== undefined) && (
+          {data.some(d => d.eventCount !== undefined) && (
             <Line 
               type="monotone" 
-              dataKey="events" 
+              dataKey="eventCount" 
               stroke="#10b981" 
               strokeWidth={2}
               name="Events"
@@ -75,6 +75,17 @@ export function TimeSeriesChart({ data, loading }: TimeSeriesChartProps) {
               activeDot={{ r: 6 }}
             />
           )}
+          {data.some(d => d.litterCollected > 0) && (
+            <Line 
+              type="monotone" 
+              dataKey="litterCollected" 
+              stroke="#f59e0b" 
+              strokeWidth={2}
+              name="Litter Collected (kg)"
+              dot={{ fill: '#f59e0b', r: 4 }}
+              activeDot={{ r: 6 }}
+            />
+          )}
         </LineChart>
       </ResponsiveContainer>
     </div>