feat: reporting dashboard — case stats, deadline compliance, workload, billing (P1)

Backend:
- ReportingService with aggregation queries (CTEs, FILTER clauses)
- 4 API endpoints: /api/reports/{cases,deadlines,workload,billing}
- Date range filtering via ?from=&to= query params

Frontend:
- /berichte page with 4 tabs: Akten, Fristen, Auslastung, Abrechnung
- recharts: bar/pie/line charts for all report types
- Date range picker, CSV export, print-friendly view
- Sidebar nav entry with BarChart3 icon

Also resolves merge conflicts between role-based, notification, and
audit trail branches, and adds missing TS types (AuditLogResponse,
Notification, NotificationPreferences).

frontend/package.json

@@ -20,6 +20,7 @@
     "react": "19.1.0",
     "react-dom": "19.1.0",
     "react-dropzone": "^15.0.0",
+    "recharts": "^3.8.1",
     "sonner": "^2.0.7"
   },
   "devDependencies": {