feat(metrics): per-staff performance view in dashboard

GET /v1/metrics/staff returns bookings, completed, cancelled,
utilization, and revenue per staff member over a trailing window
(default 30d, max 365d). Appointment bookings only — class sessions
have no staff assignment. All active staff appear, including
zero-booking rows.

Staff page gains a localized Performance panel; demo mode seeds
trailing-window bookings and matching metric rows.

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

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

apps/backend/internal/bookings/service.go

@@ -672,6 +672,51 @@ func (s *Service) DashboardSummary(ctx context.Context, principal domain.Princip
 	}, nil
 }
 
+// StaffMetrics returns per-staff appointment workload over a trailing
+// window — every active staff member appears, even with zero bookings.
+func (s *Service) StaffMetrics(ctx context.Context, principal domain.Principal, days int) (domain.StaffMetricsResponse, error) {
+	membership, err := s.repo.GetTenantMembershipBySubject(ctx, principal.Subject)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return domain.StaffMetricsResponse{}, ErrTenantMembership
+		}
+		return domain.StaffMetricsResponse{}, err
+	}
+	if days <= 0 || days > 365 {
+		days = 30
+	}
+	to := time.Now().UTC()
+	from := to.AddDate(0, 0, -days)
+
+	records, err := s.repo.GetStaffMetrics(ctx, membership.Tenant.ID, from, to)
+	if err != nil {
+		return domain.StaffMetricsResponse{}, err
+	}
+	byStaff := make(map[string]db.StaffMetricsRecord, len(records))
+	for _, rec := range records {
+		byStaff[rec.StaffID] = rec
+	}
+
+	staff, err := s.repo.ListStaffByTenant(ctx, membership.Tenant.ID)
+	if err != nil {
+		return domain.StaffMetricsResponse{}, err
+	}
+	metrics := make([]domain.StaffMetric, 0, len(staff))
+	for _, member := range staff {
+		rec := byStaff[member.ID]
+		metrics = append(metrics, domain.StaffMetric{
+			StaffID:            member.ID,
+			DisplayName:        member.DisplayName,
+			Bookings:           rec.BookingsCount,
+			Completed:          rec.CompletedCount,
+			Cancelled:          rec.CancellationsCount,
+			UtilizationPercent: rec.UtilizationPercent,
+			RevenueCents:       rec.RevenueCents,
+		})
+	}
+	return domain.StaffMetricsResponse{From: from, To: to, Staff: metrics}, nil
+}
+
 // setupCompletion scores onboarding readiness across five equal-weight checks:
 // brand profile, location, service catalog, staff roster, and schedule rules.
 func setupCompletion(brandName string, locationCount, serviceCount, staffCount, scheduleCount int) int {