feat(metrics): new-clients KPI and scheduled reminder dispatch

The dashboard promised a new-clients card the API never produced; the
metric now counts distinct customer emails whose earliest-ever booking
lands in the window, joining the other four KPIs with the same trend
comparison. The card grid rebalances for five items.

Reminder dispatch also had no trigger — the endpoint existed but
nothing called it. A scheduled GitHub Actions workflow now posts to it
every 15 minutes using the job-runner key, so reminders actually fire
once the two secrets are configured.

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

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

apps/backend/internal/db/repository.go

@@ -397,6 +397,7 @@ type DashboardMetrics struct {
 	CancellationsCount int
 	UtilizationPercent int
 	CompletedCount     int
+	NewClientsCount    int
 }
 
 type ReminderJobParams struct {
@@ -1488,6 +1489,21 @@ func (r *MemoryRepository) GetDashboardMetrics(_ context.Context, tenantID strin
 			}
 		}
 	}
+	firstSeen := map[string]time.Time{}
+	for _, booking := range r.bookings {
+		if booking.TenantID != tenantID {
+			continue
+		}
+		first, ok := firstSeen[booking.CustomerEmail]
+		if !ok || booking.StartsAt.Before(first) {
+			firstSeen[booking.CustomerEmail] = booking.StartsAt
+		}
+	}
+	for _, first := range firstSeen {
+		if !first.Before(startsAt) && first.Before(endsAt) {
+			metrics.NewClientsCount++
+		}
+	}
 	if metrics.BookingsCount == 0 {
 		metrics.UtilizationPercent = 0
 	} else {