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/db/repository.go

@@ -315,7 +315,7 @@ type StaffMetricsRecord struct {
 	BookingsCount      int
 	CancellationsCount int
 	CompletedCount     int
-	UtilizationPercent int
+	BookedMinutes      int
 	RevenueCents       int64
 }
 
@@ -1806,22 +1806,19 @@ func (r *MemoryRepository) GetStaffMetrics(_ context.Context, tenantID string, s
 		case booking.Status == "cancelled":
 			rec.CancellationsCount++
 		case booking.Status == "confirmed":
-			rec.UtilizationPercent++
 			if booking.EndsAt.Before(time.Now().UTC()) {
 				rec.CompletedCount++
 			}
 		}
-		if booking.Status != "cancelled" && booking.ServiceID != nil {
-			rec.RevenueCents += int64(servicePrices[*booking.ServiceID])
+		if booking.Status != "cancelled" {
+			rec.BookedMinutes += int(booking.EndsAt.Sub(booking.StartsAt).Minutes())
+			if booking.ServiceID != nil {
+				rec.RevenueCents += int64(servicePrices[*booking.ServiceID])
+			}
 		}
 	}
 	records := make([]StaffMetricsRecord, 0, len(byStaff))
 	for _, rec := range byStaff {
-		if rec.BookingsCount == 0 {
-			rec.UtilizationPercent = 0
-		} else {
-			rec.UtilizationPercent = int(float64(rec.UtilizationPercent) / float64(rec.BookingsCount) * 100)
-		}
 		records = append(records, *rec)
 	}
 	sort.SliceStable(records, func(i, j int) bool { return records[i].BookingsCount > records[j].BookingsCount })