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/db/bookings.go

@@ -251,6 +251,44 @@ func (r *PGRepository) GetDashboardMetrics(ctx context.Context, tenantID string,
 	return metrics, err
 }
 
+// GetStaffMetrics mirrors GetDashboardMetrics grouped by staff_id. Class
+// bookings have no staff_id and fall out naturally; revenue sums the linked
+// service price for non-cancelled bookings.
+func (r *PGRepository) GetStaffMetrics(ctx context.Context, tenantID string, startsAt time.Time, endsAt time.Time) ([]StaffMetricsRecord, error) {
+	rows, err := r.pool.Query(ctx, `
+		SELECT
+			b.staff_id,
+			COUNT(*) AS bookings_count,
+			COUNT(*) FILTER (WHERE b.status = 'cancelled') AS cancellations_count,
+			COUNT(*) FILTER (WHERE b.status = 'confirmed' AND b.ends_at < now()) AS completed_count,
+			COALESCE(
+				ROUND(100.0 * COUNT(*) FILTER (WHERE b.status = 'confirmed') / NULLIF(COUNT(*), 0)),
+				0
+			)::integer AS utilization_percent,
+			COALESCE(SUM(s.price_cents) FILTER (WHERE b.status <> 'cancelled'), 0) AS revenue_cents
+		FROM bookings b
+		LEFT JOIN services s ON s.id = b.service_id
+		WHERE b.tenant_id = $1 AND b.staff_id IS NOT NULL AND b.starts_at >= $2 AND b.starts_at < $3
+		GROUP BY b.staff_id
+		ORDER BY bookings_count DESC
+	`, tenantID, startsAt, endsAt)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var records []StaffMetricsRecord
+	for rows.Next() {
+		var rec StaffMetricsRecord
+		if err := rows.Scan(&rec.StaffID, &rec.BookingsCount, &rec.CancellationsCount,
+			&rec.CompletedCount, &rec.UtilizationPercent, &rec.RevenueCents); err != nil {
+			return nil, err
+		}
+		records = append(records, rec)
+	}
+	return records, rows.Err()
+}
+
 func (r *PGRepository) GetWaitlistPosition(ctx context.Context, sessionID, bookingID string) (int, error) {
 	var position int
 	err := r.pool.QueryRow(ctx, `