feat(reports): capacity-based utilization + report CSVs

Utilization now measures booked minutes against each member's
availability-rule capacity in the tenant timezone, replacing the
confirmed-share proxy. StaffMetric exposes booked/capacity minutes.

New endpoints: GET /v1/export/staff-report.csv (reuses StaffMetrics)
and GET /v1/export/service-report.csv (per-service bookings/cancelled/
revenue, class bookings rolled up). Export buttons on the staff
performance panel and services page.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/domain/models.go

@@ -560,12 +560,16 @@ type ActivityFeedResponse struct {
 
 // StaffMetric aggregates appointment workload for one staff member over a
 // trailing window. Class bookings carry no staff assignment and are excluded.
+// UtilizationPercent is bookedMinutes / capacityMinutes — booked time against
+// the member's availability-rule capacity in the tenant timezone.
 type StaffMetric struct {
 	StaffID            string `json:"staffId"`
 	DisplayName        string `json:"displayName"`
 	Bookings           int    `json:"bookings"`
 	Completed          int    `json:"completed"`
 	Cancelled          int    `json:"cancelled"`
+	BookedMinutes      int    `json:"bookedMinutes"`
+	CapacityMinutes    int    `json:"capacityMinutes"`
 	UtilizationPercent int    `json:"utilizationPercent"`
 	RevenueCents       int64  `json:"revenueCents"`
 }