feat: add dashboard aggregation endpoint (Phase 2I)

GET /api/dashboard returns aggregated data:
- deadline_summary: overdue, due this/next week, ok counts
- case_summary: active, new this month, closed counts
- upcoming_deadlines: next 7 days with case info
- upcoming_appointments: next 7 days
- recent_activity: last 10 case events

Uses efficient CTE query for summaries. Also fixes duplicate
writeJSON/writeError declarations in appointments handler.

backend/internal/handlers/dashboard.go

@@ -0,0 +1,32 @@
+package handlers
+
+import (
+	"net/http"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+type DashboardHandler struct {
+	svc *services.DashboardService
+}
+
+func NewDashboardHandler(svc *services.DashboardService) *DashboardHandler {
+	return &DashboardHandler{svc: svc}
+}
+
+func (h *DashboardHandler) Get(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	data, err := h.svc.Get(r.Context(), tenantID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	writeJSON(w, http.StatusOK, data)
+}