Fix Maloja importer resilience: nil MBZ client, null album, error handling

Root cause of the panic at ~1,693 items: importers created
`&mbz.MusicBrainzClient{}` (nil requestQueue) instead of using the
engine's properly initialized client. When any code path called an MBZ
method, it panicked on the nil channel.

Changes:
- Pass engine's MBZ client to Maloja and Spotify importers
- Change MalojaTrack.Album to pointer type to handle null album JSON
- Continue on error instead of aborting the entire import
- Accept both Maloja export formats ("scrobbles" and "list" keys)
- Extract per-file import into importFile() with its own defer/recover
- Add progress logging every 500 items

Test fixtures:
- maloja_import_null_album_test.json (null album, valid album, empty artists)
- maloja_api_format_test.json (API "list" format)

New tests: TestImportMaloja_NullAlbum, TestImportMaloja_ApiFormat

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

internal/importer/spotify.go

@@ -24,7 +24,7 @@ type SpotifyExportItem struct {
 	MsPlayed   int32     `json:"ms_played"`
 }
 
-func ImportSpotifyFile(ctx context.Context, store db.DB, filename string) error {
+func ImportSpotifyFile(ctx context.Context, store db.DB, mbzc mbz.MusicBrainzCaller, filename string) error {
 	l := logger.FromContext(ctx)
 	l.Info().Msgf("Beginning spotify import on file: %s", filename)
 	file, err := os.Open(path.Join(cfg.ConfigDir(), "import", filename))
@@ -59,7 +59,7 @@ func ImportSpotifyFile(ctx context.Context, store db.DB, filename string) error
 			continue
 		}
 		opts := catalog.SubmitListenOpts{
-			MbzCaller:      &mbz.MusicBrainzClient{},
+			MbzCaller:      mbzc,
 			Artist:         item.ArtistName,
 			TrackTitle:     item.TrackName,
 			ReleaseTitle:   item.AlbumName,