- Add _extract_discogs_fields to metadata cache — handles Discogs field
names (title vs name, images array, Artist - Title format)
- Worker uses _fetch_and_cache_artist/_fetch_and_cache_album helpers
that cache raw data while returning it for enrichment
- All search/lookup methods cache results for repeat queries
- Cache browser: Discogs stat pill, source filter, clear button, badge
- Fixes albums showing as 'Unknown' and artists missing images in cache
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