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/db/bookings.go

@@ -181,7 +181,16 @@ func (r *PGRepository) GetDashboardMetrics(ctx context.Context, tenantID string,
 				),
 				0
 			)::integer AS utilization_percent,
-			COUNT(*) FILTER (WHERE starts_at >= $2 AND starts_at < $3 AND status = 'confirmed' AND ends_at < now()) AS completed_count
+			COUNT(*) FILTER (WHERE starts_at >= $2 AND starts_at < $3 AND status = 'confirmed' AND ends_at < now()) AS completed_count,
+			COUNT(DISTINCT customer_email) FILTER (
+				WHERE starts_at >= $2 AND starts_at < $3
+					AND NOT EXISTS (
+						SELECT 1 FROM bookings earlier
+						WHERE earlier.tenant_id = bookings.tenant_id
+							AND earlier.customer_email = bookings.customer_email
+							AND earlier.starts_at < $2
+					)
+			) AS new_clients_count
 		FROM bookings
 		WHERE tenant_id = $1
 	`, tenantID, startsAt, endsAt).Scan(
@@ -189,6 +198,7 @@ func (r *PGRepository) GetDashboardMetrics(ctx context.Context, tenantID string,
 		&metrics.CancellationsCount,
 		&metrics.UtilizationPercent,
 		&metrics.CompletedCount,
+		&metrics.NewClientsCount,
 	)
 	return metrics, err
 }