feat(library): surface metadata enrichment status, errors and covers

The mapper dropped metadata_status and hardcoded availability, so enrichment
state was invisible and a just-uploaded track never appeared to change. Map
metadata_status/metadata_error/has_cover onto Track; add MetadataStatusBadge
(pending spinner / enriched / failed-with-reason / manual) shown in TrackRow,
and serve token-bearing track covers via getTrackCoverUrl.

UploadPage now polls each uploaded track (stops once enrichment settles) so the
resolved title/artist — or a failure reason — appears live. i18n in en + ru.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/i18n/locales/en.ts

@@ -230,6 +230,20 @@ const en = {
       error: 'Failed',
     },
   },
+  metadata: {
+    status: {
+      pending: 'Enriching…',
+      enriched: 'Enriched',
+      failed: 'No match',
+      manual: 'Manual',
+    },
+    statusHint: {
+      pending: 'Identifying metadata…',
+      enriched: 'Metadata identified',
+      failed: 'Metadata could not be identified',
+      manual: 'Edited manually — not auto-updated',
+    },
+  },
 } as const;
 
 export default en;