fix(api): use UTC methods in defaultFrom/defaultTo date helpers

The server-side date fallback helpers used local-timezone JS Date
methods (setDate/setHours), creating a mismatch when clients send
explicit UTC dates (e.g. 2026-02-28T00:00:00Z). Changed to UTC
methods (setUTCDate/setUTCHours) to ensure consistent UTC behavior.

Also fixed the 90-day churn-risk lookback to use UTC consistently.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/routes/reports.ts

@@ -31,14 +31,14 @@ function parseDate(value: string | undefined, fallback: Date): Date {
 
 function defaultFrom(): Date {
   const d = new Date();
-  d.setDate(d.getDate() - 30);
-  d.setHours(0, 0, 0, 0);
+  d.setUTCDate(d.getUTCDate() - 30);
+  d.setUTCHours(0, 0, 0, 0);
   return d;
 }
 
 function defaultTo(): Date {
   const d = new Date();
-  d.setHours(23, 59, 59, 999);
+  d.setUTCHours(23, 59, 59, 999);
   return d;
 }
 
@@ -283,7 +283,7 @@ reportsRouter.get("/clients", async (c) => {
 
   // Clients with no appointment in last 90 days (churn risk)
   const ninetyDaysAgo = new Date();
-  ninetyDaysAgo.setDate(ninetyDaysAgo.getDate() - 90);
+  ninetyDaysAgo.setUTCDate(ninetyDaysAgo.getUTCDate() - 90);
   const ninetyDaysAgoISO = ninetyDaysAgo.toISOString();
 
   const churnRisk = await db