Fix reports crash: serialize Date as ISO string in churn risk query

The /api/reports/clients endpoint crashes with a 500 because
Drizzle's sql template literal in a HAVING clause cannot serialize
a JavaScript Date object — the postgres driver expects a string.

Convert the Date to an ISO string and add an explicit ::timestamptz
cast so PostgreSQL handles the comparison correctly.

Closes groombook/groombook#49

Co-Authored-By: Paperclip <noreply@paperclip.ing>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/routes/reports.ts

@@ -279,6 +279,7 @@ reportsRouter.get("/clients", async (c) => {
   // Clients with no appointment in last 90 days (churn risk)
   const ninetyDaysAgo = new Date();
   ninetyDaysAgo.setDate(ninetyDaysAgo.getDate() - 90);
+  const ninetyDaysAgoISO = ninetyDaysAgo.toISOString();
 
   const churnRisk = await db
     .select({
@@ -290,7 +291,7 @@ reportsRouter.get("/clients", async (c) => {
     .leftJoin(appointments, eq(appointments.clientId, clients.id))
     .groupBy(clients.id, clients.name)
     .having(
-      sql`MAX(${appointments.startTime}) < ${ninetyDaysAgo} OR MAX(${appointments.startTime}) IS NULL`
+      sql`MAX(${appointments.startTime}) < ${ninetyDaysAgoISO}::timestamptz OR MAX(${appointments.startTime}) IS NULL`
     )
     .orderBy(sql`MAX(${appointments.startTime}) ASC NULLS FIRST`);