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.

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

apps/backend/internal/bookings/service.go

@@ -626,6 +626,7 @@ func (s *Service) DashboardSummary(ctx context.Context, principal domain.Princip
 			dashboardKPI("completed", "Completed", previousMetrics.CompletedCount, priorMetrics.CompletedCount, ""),
 			dashboardKPI("cancellations", "Cancellations", metrics.CancellationsCount, previousMetrics.CancellationsCount, ""),
 			dashboardKPI("utilization", "Utilization", metrics.UtilizationPercent, previousMetrics.UtilizationPercent, "%"),
+			dashboardKPI("new_clients", "New clients", metrics.NewClientsCount, previousMetrics.NewClientsCount, ""),
 		},
 		UpcomingBookings: upcoming,
 		AllBookings:      allBookings,