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).

backend/internal/handlers/deadlines.go

@@ -198,18 +198,8 @@ func (h *DeadlineHandlers) Delete(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-<<<<<<< HEAD
-	if err := h.deadlines.Delete(tenantID, deadlineID); err != nil {
+	if err := h.deadlines.Delete(r.Context(), tenantID, deadlineID); err != nil {
 		writeError(w, http.StatusNotFound, "deadline not found")
-||||||| 82878df
-	err = h.deadlines.Delete(tenantID, deadlineID)
-	if err != nil {
-		writeError(w, http.StatusNotFound, err.Error())
-=======
-	err = h.deadlines.Delete(r.Context(), tenantID, deadlineID)
-	if err != nil {
-		writeError(w, http.StatusNotFound, err.Error())
->>>>>>> mai/knuth/p0-audit-trail-append
 		return
 	}