fix(library): preserve playlist phantoms on incremental scan and removal

The incremental scan's orphan cleanup and RemoveFromLibrary deleted
audio_files rows without first filling the playlist phantom columns, so
a track removed from the library folder outside YellowJacket (or
removed from the library) became a permanently empty playlist row that
nothing could re-link — the same bug #183 fixed on the full-rescan and
retire paths, on the two paths it missed.

Add a scoped PreservePlaylistPhantomsForFiles and run it in the same
transaction as the deletes on both paths.

Closes #246

backend/database/playlistphantoms_test.go

@@ -0,0 +1,94 @@
+package database
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+)
+
+// TestPreservePlaylistPhantomsForFilesScopesToTheRequestedIDs is the
+// scoping half of the scoped variant: a run over one file's id must
+// fill that file's playlist entries and leave every other entry alone,
+// because the incremental scan calls this once per orphan batch and a
+// pass that rewrote the whole table would touch every playlist row on
+// every scan.
+func TestPreservePlaylistPhantomsForFilesScopesToTheRequestedIDs(t *testing.T) {
+	ctx := context.Background()
+	db := openRaw(t, t.TempDir())
+
+	if _, err := db.ExecContext(ctx, "PRAGMA foreign_keys = ON"); err != nil {
+		t.Fatalf("pragma: %v", err)
+	}
+
+	if err := applySchema(ctx, db); err != nil {
+		t.Fatalf("applySchema: %v", err)
+	}
+
+	if _, err := db.ExecContext(ctx, `
+		INSERT INTO playlists (id, name) VALUES (1, 'keepme');
+		INSERT INTO libraries (id, name, path) VALUES (0, 'test', '/music');
+		INSERT INTO artists (id, name) VALUES (3, 'Aurora Fields');
+		INSERT INTO cover_art (id, file_path, mime_type)
+			VALUES (9, 'covers/7.jpg', 'image/jpeg');
+		INSERT INTO albums (id, name, artist_id, cover_art_id)
+			VALUES (4, 'Tideline', 3, 9);
+		INSERT INTO audio_files
+			(id, file_path, file_type_id, length_milliseconds,
+			 title, artist_credit, artist_id, album_id)
+			VALUES
+				(7, '/music/a.flac', 1, 1000,
+				 'Slack Water', 'Aurora Fields', 3, 4),
+				(8, '/music/b.flac', 1, 2000,
+				 'Second Tide', 'Aurora Fields', 3, 4);
+		INSERT INTO playlist_tracks (playlist_id, audio_file_id, position)
+			VALUES (1, 7, 0), (1, 8, 1);
+	`); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		t.Fatalf("begin: %v", err)
+	}
+
+	defer func() { _ = tx.Rollback() }()
+
+	if err := PreservePlaylistPhantomsForFiles(
+		ctx, tx, []int64{7}, testLogger(),
+	); err != nil {
+		t.Fatalf("preserve: %v", err)
+	}
+
+	if err := tx.Commit(); err != nil {
+		t.Fatalf("commit: %v", err)
+	}
+
+	var filled, untouched sql.NullString
+
+	if err := db.QueryRowContext(ctx,
+		"SELECT phantom_file_path FROM playlist_tracks WHERE audio_file_id = 7",
+	).Scan(&filled); err != nil {
+		t.Fatalf("read the requested entry: %v", err)
+	}
+
+	if filled.String != "/music/a.flac" {
+		t.Errorf(
+			"requested entry phantom_file_path = %q, want %q",
+			filled.String, "/music/a.flac",
+		)
+	}
+
+	if err := db.QueryRowContext(ctx,
+		"SELECT phantom_file_path FROM playlist_tracks WHERE audio_file_id = 8",
+	).Scan(&untouched); err != nil {
+		t.Fatalf("read the untouched entry: %v", err)
+	}
+
+	if untouched.Valid {
+		t.Errorf(
+			"untouched entry got phantom_file_path = %q, want NULL "+
+				"(a scoped run must not rewrite the whole table)",
+			untouched.String,
+		)
+	}
+}