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/api/endpoints/streaming.ts

@@ -17,3 +17,18 @@ export function getCoverUrl(artUrl: string | undefined): string | undefined {
   const base = getApiBaseUrl();
   return `${base}${artUrl}`;
 }
+
+/**
+ * Cover image URL for a track, served by `GET /tracks/{id}/cover`. Like the
+ * audio stream, an `<img>` can't send an `Authorization` header, so the access
+ * token rides as `?token=`. Returns undefined when the track has no cover.
+ */
+export function getTrackCoverUrl(
+  trackId: string,
+  token: string,
+  hasCover: boolean,
+): string | undefined {
+  if (!hasCover) return undefined;
+  const base = getApiBaseUrl();
+  return `${base}/tracks/${trackId}/cover?token=${encodeURIComponent(token)}`;
+}