feat(thumb-backfill): Pinia store — triggerBackfill() POSTs /api/thumbnails/backfill

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/thumbnails.js

@@ -0,0 +1,12 @@
+import { defineStore } from 'pinia'
+import { useApi } from '../composables/useApi.js'
+
+export const useThumbnailsStore = defineStore('thumbnails', () => {
+  const api = useApi()
+
+  async function triggerBackfill () {
+    await api.post('/api/thumbnails/backfill')
+  }
+
+  return { triggerBackfill }
+})