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/frontend/src/routes/dashboard/dashboard-data.ts

@@ -216,6 +216,7 @@ export function DashboardDataProvider(props: { children: any }) {
     completed: "dashboard.kpi.completed",
     cancellations: "dashboard.kpi.cancellations",
     utilization: "dashboard.kpi.utilization",
+    new_clients: "dashboard.kpi.newClients",
   };
   const normalizedKpis = createMemo(() => (resolvedSummary()?.kpis ?? []).map((kpi: any) => ({
     ...kpi,