Add bulk import optimization: track_lookup cache, batch inserts, BulkSubmitter

Adopts ListenBrainz-inspired patterns to speed up imports from ~24h to
under 30 minutes for 49k scrobbles.

Phase 1 - track_lookup cache table:
- New migration (000006) adds persistent entity lookup cache
- Maps normalized (artist, track, album) → (artist_id, album_id, track_id)
- SubmitListen fast path: cache hit skips 18 DB queries → 2 queries
- Cache populated after entity resolution, invalidated on merge/delete
- Benefits both live scrobbles and imports

Phase 2 - SaveListensBatch:
- New batch listen insert using pgx CopyFrom → temp table → INSERT ON CONFLICT
- Thousands of inserts per second vs one-at-a-time

Phase 3 - BulkSubmitter:
- Reusable import accelerator for all importers
- Pre-deduplicates scrobbles by (artist, track, album) in memory
- Worker pool (4 goroutines) for parallel entity creation on cache miss
- Batch listen insertion via SaveListensBatch

Phase 4 - Migrate importers:
- Maloja, Spotify, LastFM, ListenBrainz importers use BulkSubmitter
- Koito importer left as-is (already fast with pre-resolved IDs)

Phase 5 - Skip image lookups during import:
- GetArtistImage/GetAlbumImage calls fully skipped when SkipCacheImage=true
- Background tasks (FetchMissingArtistImages/FetchMissingAlbumImages) backfill

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/db/db.go

@@ -101,6 +101,16 @@ type DB interface {
 	MergeAlbums(ctx context.Context, fromId, toId int32, replaceImage bool) error
 	MergeArtists(ctx context.Context, fromId, toId int32, replaceImage bool) error
 
+	// Track Lookup Cache
+
+	GetTrackLookup(ctx context.Context, key string) (*TrackLookupResult, error)
+	SaveTrackLookup(ctx context.Context, opts SaveTrackLookupOpts) error
+	InvalidateTrackLookup(ctx context.Context, opts InvalidateTrackLookupOpts) error
+
+	// Batch
+
+	SaveListensBatch(ctx context.Context, opts []SaveListenOpts) (int64, error)
+
 	// Etc
 
 	ImageHasAssociation(ctx context.Context, image uuid.UUID) (bool, error)