fix UI 503s under checkpoint load

- Store: skip identical CT checkpoint writes (CertSpotter calls StoreLogState
  per batch across 64 logs; hashing the state avoids redundant writes).
- webui: cache /api/stats for 10s and raise API query timeouts; UI polls, so
  repeated aggregate queries no longer queue behind checkpoint bursts.
- reword discovery detail (no 'name enumeration' for S3/GCS; only Azure uses a
  container list).

internal/metrics/metrics.go

@@ -90,7 +90,7 @@ func (m *Metrics) Handler() http.Handler {
 		counter("probe_public_total", "Probes classified as public read", m.ProbePublic.Load())
 		counter("entries_indexed_total", "Metadata entries indexed from readable resources", m.EntriesIndexed.Load())
 		counter("bucket_candidates_total", "Candidate bucket names tested", m.BucketCandidates.Load())
-		counter("bucket_open_total", "Open (listable) buckets discovered by name enumeration", m.BucketOpen.Load())
+		counter("bucket_open_total", "Open (listable) buckets discovered from CT hostname labels", m.BucketOpen.Load())
 		for _, g := range m.gauges {
 			fmt.Fprintf(w, "# HELP %s %s\n# TYPE %s gauge\n%s%s %g\n", g.name, g.help, g.name, g.name, g.extra, g.fn())
 		}