Add paginated library JSON and dedicated still thumbs that never stream archive masters.

Co-authored-by: Cursor <cursoragent@cursor.com>

components/studio2/AssetSlot.vue

@@ -55,6 +55,6 @@ defineEmits(['pick', 'remove', 'mention', 'update:role'])
 const thumbSrc = computed(() => {
  if (!props.asset) return ''
  if (props.asset.kind === 'clip') return `/api/library/clips/${props.asset.id}/thumb`
- return `/api/library/stills/${props.asset.id}`
+ return `/api/library/stills/${props.asset.id}/thumb`
 })
 </script>