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_test.go

@@ -766,3 +766,77 @@ func TestWaitlistPositionReflectsQueueOrder(t *testing.T) {
 		t.Fatalf("expected promoted booking to report no position, got %d", got)
 	}
 }
+
+func TestStaffMetricsAggregatesPerStaff(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	service := NewService(repo, nil, "http://localhost:3000")
+	ctx := context.Background()
+
+	tenantID := "5d6b3551-0a3e-4b86-bdf0-e9df20a47148"
+	staffID := "6936c444-c7d0-4a7d-b596-a9b72d2f4fc0"   // Demo Stylist
+	serviceID := "d5d76a61-3d49-467c-8dd4-bf61ee754e39" // Signature treatment, 1200 Kč
+	now := time.Now().UTC()
+
+	seed := func(offsetDays int, status string, withService bool) {
+		t.Helper()
+		params := db.CreateBookingParams{
+			TenantID:      tenantID,
+			StaffID:       &staffID,
+			CustomerName:  "Client",
+			CustomerEmail: "[email protected]",
+			StartsAt:      now.AddDate(0, 0, offsetDays),
+			EndsAt:        now.AddDate(0, 0, offsetDays).Add(time.Hour),
+			Status:        status,
+			Reference:     "BK-SM",
+		}
+		if withService {
+			params.ServiceID = &serviceID
+		}
+		if _, err := repo.CreateBooking(ctx, params); err != nil {
+			t.Fatalf("seed booking: %v", err)
+		}
+	}
+
+	seed(-5, "confirmed", true)
+	seed(-3, "confirmed", true)   // already ended → completed too
+	seed(-2, "cancelled", false)  // cancelled → no revenue
+	seed(-40, "confirmed", true)  // outside the 30-day window
+	// Class booking (no staff) must not appear in per-staff metrics.
+	sessionID := "4bf74c12-44dd-45ca-86bb-b104f16f2435"
+	if _, err := repo.CreateBooking(ctx, db.CreateBookingParams{
+		TenantID:       tenantID,
+		ClassSessionID: &sessionID,
+		CustomerName:   "Class Client",
+		CustomerEmail:  "[email protected]",
+		StartsAt:       now.AddDate(0, 0, -1),
+		EndsAt:         now.AddDate(0, 0, -1).Add(time.Hour),
+		Status:         "confirmed",
+		Reference:      "BK-SM-CLASS",
+	}); err != nil {
+		t.Fatalf("seed class booking: %v", err)
+	}
+
+	response, err := service.StaffMetrics(ctx, domain.Principal{Subject: "demo-owner", Role: "owner"}, 30)
+	if err != nil {
+		t.Fatalf("staff metrics: %v", err)
+	}
+	if len(response.Staff) != 1 {
+		t.Fatalf("expected 1 staff row, got %d", len(response.Staff))
+	}
+	row := response.Staff[0]
+	if row.StaffID != staffID || row.DisplayName != "Demo Stylist" {
+		t.Fatalf("unexpected staff row identity: %+v", row)
+	}
+	if row.Bookings != 3 {
+		t.Fatalf("expected 3 bookings in window, got %d", row.Bookings)
+	}
+	if row.Cancelled != 1 {
+		t.Fatalf("expected 1 cancellation, got %d", row.Cancelled)
+	}
+	if row.UtilizationPercent != 66 {
+		t.Fatalf("expected 66%% utilization, got %d", row.UtilizationPercent)
+	}
+	if row.RevenueCents != 240000 {
+		t.Fatalf("expected 240000 revenue cents (2×1200 Kč), got %d", row.RevenueCents)
+	}
+}