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/types.ts

@@ -1,5 +1,13 @@
 export type TrackAvailability = 'server' | 'downloading' | 'error' | 'missing';
 
+/**
+ * Metadata-enrichment state, distinct from file `availability`. `pending` = the
+ * worker hasn't finished (or hasn't started); `enriched` = identity found;
+ * `failed` = no match / a worker error (see `metadataError`); `manual` = user-
+ * edited and never auto-overwritten.
+ */
+export type MetadataStatus = 'pending' | 'enriched' | 'failed' | 'manual';
+
 export interface Track {
   id: string;
   title: string;
@@ -8,12 +16,16 @@ export interface Track {
   albumId: string;
   albumTitle: string;
   albumArtUrl?: string;
+  hasCover: boolean;
   durationMs: number;
   trackNumber?: number;
   discNumber?: number;
   year?: number;
   genre?: string;
   availability: TrackAvailability;
+  metadataStatus: MetadataStatus;
+  /** Human-readable reason the last enrichment run set `failed`; else undefined. */
+  metadataError?: string;
   fileSize?: number;
   format?: string;
   bitrate?: number;