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/reports.go

@@ -0,0 +1,109 @@
+package handlers
+
+import (
+	"net/http"
+	"time"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+type ReportHandler struct {
+	svc *services.ReportingService
+}
+
+func NewReportHandler(svc *services.ReportingService) *ReportHandler {
+	return &ReportHandler{svc: svc}
+}
+
+// parseDateRange extracts from/to query params, defaulting to last 12 months.
+func parseDateRange(r *http.Request) (time.Time, time.Time) {
+	now := time.Now()
+	from := time.Date(now.Year()-1, now.Month(), 1, 0, 0, 0, 0, time.UTC)
+	to := time.Date(now.Year(), now.Month(), now.Day(), 23, 59, 59, 0, time.UTC)
+
+	if v := r.URL.Query().Get("from"); v != "" {
+		if t, err := time.Parse("2006-01-02", v); err == nil {
+			from = t
+		}
+	}
+	if v := r.URL.Query().Get("to"); v != "" {
+		if t, err := time.Parse("2006-01-02", v); err == nil {
+			to = time.Date(t.Year(), t.Month(), t.Day(), 23, 59, 59, 0, time.UTC)
+		}
+	}
+
+	return from, to
+}
+
+func (h *ReportHandler) Cases(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	from, to := parseDateRange(r)
+
+	data, err := h.svc.CaseReport(r.Context(), tenantID, from, to)
+	if err != nil {
+		internalError(w, "failed to generate case report", err)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, data)
+}
+
+func (h *ReportHandler) Deadlines(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	from, to := parseDateRange(r)
+
+	data, err := h.svc.DeadlineReport(r.Context(), tenantID, from, to)
+	if err != nil {
+		internalError(w, "failed to generate deadline report", err)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, data)
+}
+
+func (h *ReportHandler) Workload(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	from, to := parseDateRange(r)
+
+	data, err := h.svc.WorkloadReport(r.Context(), tenantID, from, to)
+	if err != nil {
+		internalError(w, "failed to generate workload report", err)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, data)
+}
+
+func (h *ReportHandler) Billing(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	from, to := parseDateRange(r)
+
+	data, err := h.svc.BillingReport(r.Context(), tenantID, from, to)
+	if err != nil {
+		internalError(w, "failed to generate billing report", err)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, data)
+}