fix(lyrics): preserve [] (not null) for empty lyrics in backfill migration

ParseLyrics returns nil for zero-line input (whitespace-only stored
lyrics). json.Marshal(nil LyricList) produces null, violating the DB
invariant that media_file.lyrics uses [] for empty lyrics, never null.
Initialize to model.LyricList{} when ParseLyrics returns nil so the
marshalled result is always [].

db/migrations/20231209211223_alter_lyric_column.go

@@ -50,6 +50,10 @@ func upAlterLyricColumn(ctx context.Context, tx *sql.Tx) error {
 		if err != nil {
 			return err
 		}
+		if parsed == nil {
+			// Preserve the column invariant: empty lyrics serialize to [], never null
+			parsed = model.LyricList{}
+		}
 
 		text, err := json.Marshal(parsed)
 		if err != nil {