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/api/server.go

@@ -1296,6 +1296,20 @@ func NewServer(cfg config.Config, pools *db.Pools) (*Server, error) {
 		c.JSON(http.StatusOK, response)
 	})
 
+	protected.GET("/metrics/staff", func(c *gin.Context) {
+		days, _ := strconv.Atoi(c.DefaultQuery("days", "30"))
+		response, err := bookingService.StaffMetrics(c.Request.Context(), auth.PrincipalFromContext(c), days)
+		if err != nil {
+			status := http.StatusInternalServerError
+			if errors.Is(err, bookings.ErrTenantMembership) {
+				status = http.StatusNotFound
+			}
+			c.JSON(status, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, response)
+	})
+
 	protected.GET("/export/bookings.csv", func(c *gin.Context) {
 		from, to := exportRange(c)
 		var buf bytes.Buffer