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>

db/migrations/000006_track_lookup.sql

@@ -0,0 +1,15 @@
+-- +goose Up
+CREATE TABLE track_lookup (
+    lookup_key TEXT NOT NULL PRIMARY KEY,
+    artist_id INT NOT NULL REFERENCES artists(id) ON DELETE CASCADE,
+    album_id INT NOT NULL REFERENCES releases(id) ON DELETE CASCADE,
+    track_id INT NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_track_lookup_track_id ON track_lookup(track_id);
+CREATE INDEX idx_track_lookup_artist_id ON track_lookup(artist_id);
+CREATE INDEX idx_track_lookup_album_id ON track_lookup(album_id);
+
+-- +goose Down
+DROP TABLE IF EXISTS track_lookup;