feat(fc3c): downloads Pinia store + sources.checkNow action + vitests

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

frontend/src/stores/sources.js

@@ -70,6 +70,20 @@ export const useSourcesStore = defineStore('sources', () => {
     return await api.get('/api/artists/autocomplete', { params: { q: query, limit } })
   }
 
+  // FC-3c: trigger a download for one source. Returns {download_event_id,status}.
+  const checkingIds = ref(new Set())
+
+  async function checkNow(id) {
+    checkingIds.value = new Set(checkingIds.value).add(id)
+    try {
+      return await api.post(`/api/sources/${id}/check`)
+    } finally {
+      const next = new Set(checkingIds.value)
+      next.delete(id)
+      checkingIds.value = next
+    }
+  }
+
   function sourcesByArtistGrouped() {
     // returns [{artist: {id,name,slug}, sources: [...]}, ...]
     const arr = byArtist.value.get(null) ?? []
@@ -92,8 +106,10 @@ export const useSourcesStore = defineStore('sources', () => {
   return {
     byArtist, loading, error,
     allSources,
+    checkingIds,
     loadAll, loadForArtist,
     create, update, remove,
+    checkNow,
     findOrCreateArtist, autocompleteArtist,
     sourcesByArtistGrouped,
   }