fix(uats): never block on the feature-count groupBy — cold cache froze UAT selector

The ~30s groupBy over 9.7M GisFeature rows ran synchronously on the first
/api/eterra/uats call after every redeploy (in-memory cache), freezing the
UAT autocomplete right when users reload post-deploy. Counts only feed the
decorative 'N local' badge — return the (possibly empty) cache immediately
and refresh in the background, single-flight so concurrent cold requests
don't stack 30s queries.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/api/eterra/uats/route.ts

@@ -15,27 +15,29 @@ export const dynamic = "force-dynamic";
 
 const gCache = globalThis as {
   __featureCountCache?: { map: Map<string, number>; ts: number };
+  __featureCountInFlight?: Promise<void> | null;
 };
 
-async function getCachedFeatureCounts(): Promise<Map<string, number>> {
+// NEVER block the response on the counts query — the groupBy takes ~30s
+// on 9.7M GisFeature rows and the in-memory cache dies on every redeploy,
+// which used to freeze the UAT selector for the first ~30s after a deploy
+// (2026-06-04 incident). Counts only feed the decorative "N local" badge;
+// a cold start simply renders the badge as absent until the first refresh
+// lands. Single-flight so concurrent cold requests don't pile up 30s
+// queries on the DB.
+function getCachedFeatureCounts(): Map<string, number> {
   const TTL = 5 * 60 * 1000; // 5 minutes
-  const now = Date.now();
+  const cached = gCache.__featureCountCache;
 
-  if (gCache.__featureCountCache && now - gCache.__featureCountCache.ts < TTL) {
+  if (!cached || Date.now() - cached.ts >= TTL) {
-    return gCache.__featureCountCache.map;
-  }
-
-  // Run in background if cache exists but expired (return stale, refresh async)
-  if (gCache.__featureCountCache) {
     void refreshFeatureCounts();
-    return gCache.__featureCountCache.map;
+  }
+  return cached?.map ?? new Map();
 }
 
-  // First call: must wait
+function refreshFeatureCounts(): Promise<void> {
-  return refreshFeatureCounts();
+  if (gCache.__featureCountInFlight) return gCache.__featureCountInFlight;
-}
+  gCache.__featureCountInFlight = (async () => {
-
-async function refreshFeatureCounts(): Promise<Map<string, number>> {
     try {
       const groups = await prisma.gisFeature.groupBy({
         by: ["siruta"],
@@ -46,10 +48,13 @@ async function refreshFeatureCounts(): Promise<Map<string, number>> {
         map.set(g.siruta, g._count.id);
       }
       gCache.__featureCountCache = { map, ts: Date.now() };
-    return map;
     } catch {
-    return gCache.__featureCountCache?.map ?? new Map();
+      // keep whatever cache we had; next request retries
+    } finally {
+      gCache.__featureCountInFlight = null;
     }
+  })();
+  return gCache.__featureCountInFlight;
 }
 
 /* ------------------------------------------------------------------ */
@@ -171,10 +176,9 @@ export async function GET() {
       select: { siruta: true, name: true, county: true, workspacePk: true },
     });
 
-    // Feature counts: use in-memory cache (refreshed every 5 min)
+    // Feature counts: in-memory cache, refreshed in the background (never
-    // The groupBy query is expensive (~25s without cache) but the data
+    // awaited — see getCachedFeatureCounts). Cold cache → badge absent.
-    // changes rarely (only when sync jobs run)
+    const featureCounts = getCachedFeatureCounts();
-    const featureCounts = await getCachedFeatureCounts();
 
     const uats: UatResponse[] = rows.map((r) => ({
       siruta: r.siruta,