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/domain/models.go

@@ -556,6 +556,24 @@ type ActivityFeedResponse struct {
 	Events []ActivityEvent `json:"events"`
 }
 
+// StaffMetric aggregates appointment workload for one staff member over a
+// trailing window. Class bookings carry no staff assignment and are excluded.
+type StaffMetric struct {
+	StaffID            string `json:"staffId"`
+	DisplayName        string `json:"displayName"`
+	Bookings           int    `json:"bookings"`
+	Completed          int    `json:"completed"`
+	Cancelled          int    `json:"cancelled"`
+	UtilizationPercent int    `json:"utilizationPercent"`
+	RevenueCents       int64  `json:"revenueCents"`
+}
+
+type StaffMetricsResponse struct {
+	From  time.Time     `json:"from"`
+	To    time.Time     `json:"to"`
+	Staff []StaffMetric `json:"staff"`
+}
+
 // ============================================
 // ADMIN MODELS
 // ============================================