fix: address observability review feedback

src/server/routes/api/stats.siteStatus.test.ts

@@ -315,6 +315,7 @@ describe('stats dashboard filters disabled sites', () => {
         availabilityPercent: number | null;
         averageLatencyMs: number | null;
         buckets: Array<{
+          startUtc?: string;
           totalRequests: number;
           successCount: number;
           failedCount: number;
@@ -343,5 +344,6 @@ describe('stats dashboard filters disabled sites', () => {
     expect(
       body.siteAvailability?.[0]?.buckets.reduce((sum, bucket) => sum + bucket.failedCount, 0),
     ).toBe(1);
+    expect(typeof body.siteAvailability?.[0]?.buckets[0]?.startUtc).toBe('string');
   });
 });

src/server/routes/api/stats.ts

@@ -183,6 +183,7 @@ type SiteAvailabilityLogRow = {
 };
 
 type SiteAvailabilityBucketAccumulator = {
+  startUtc: string;
   label: string;
   totalRequests: number;
   successCount: number;
@@ -210,6 +211,7 @@ function buildSiteAvailabilitySummaries(
     Array.from({ length: SITE_AVAILABILITY_BUCKET_COUNT }, (_, index) => {
       const bucketStart = new Date(startMs + index * SITE_AVAILABILITY_BUCKET_MS);
       return {
+        startUtc: bucketStart.toISOString(),
         label: formatLocalDateTime(bucketStart),
         totalRequests: 0,
         successCount: 0,
@@ -293,6 +295,7 @@ function buildSiteAvailabilitySummaries(
         ? Math.round(aggregate.latencyTotalMs / aggregate.latencyCount)
         : null,
       buckets: aggregate.buckets.map((bucket) => ({
+        startUtc: bucket.startUtc,
         label: bucket.label,
         totalRequests: bucket.totalRequests,
         successCount: bucket.successCount,