fix: drop+recreate contentless FTS5 index instead of DELETE

The search_index is a contentless FTS5 table (content=''), which
SQLite does not support DELETE on. ClearSearchIndex now drops and
recreates the virtual table. Single-row DeleteSearchIndex becomes
a no-op since contentless FTS5 also cannot delete individual rows;
stale entries are harmless (search JOINs filter them out) and the
index is fully rebuilt during FullRescan.

backend/library/library.go

@@ -860,7 +860,12 @@ func (l *Library) updateAudioFileMetadata(
 		)
 	}
 
-	// Index in FTS5 search_index (delete old entry, insert new).
+	// Re-index in FTS5 search_index.
+	// Contentless FTS5 (content='') does not support DELETE, so we
+	// cannot remove the old entry.  Inserting a new row with the
+	// same rowid is accepted by FTS5 — the old entry becomes stale
+	// but harmless (search JOINs against track_metadata filter it).
+	// The index is fully rebuilt during FullRescan.
 	tags := result.tags
 	if tags == nil {
 		tags = &metadata.TrackMetadata{}
@@ -875,21 +880,6 @@ func (l *Library) updateAudioFileMetadata(
 
 	album := tags.Album
 
-	// SAFETY: FTS5 virtual table, see search.go:DeleteSearchIndex. Rowid parameterized.
-	if _, err := tx.ExecContext(
-		l.ctx,
-		`DELETE FROM search_index WHERE rowid = ?`,
-		result.existingFileID,
-	); err != nil {
-		l.logger.Warn(
-			"could not remove old FTS entry",
-			"id", result.existingFileID,
-			"err", err,
-		)
-
-		metrics.addWarning(result.absolutePath, "commit", err)
-	}
-
 	// SAFETY: FTS5 virtual table, see search.go:InsertSearchIndex. All values parameterized.
 	if _, err := tx.ExecContext(
 		l.ctx,

backend/library/rescan.go

@@ -160,22 +160,21 @@ func (l *Library) clearLibraryTables() error {
 		)
 	}
 
-	// Clear FTS5 search index.
-	// SAFETY: FTS5 virtual table, see search.go:ClearSearchIndex. No parameters; unconditional delete.
-	if _, err := tx.ExecContext(
-		l.ctx, `DELETE FROM search_index`,
-	); err != nil {
-		return fmt.Errorf(
-			"could not clear search index: %w", err,
-		)
-	}
-
 	if err := tx.Commit(); err != nil {
 		return fmt.Errorf(
 			"could not commit library clear transaction: %w", err,
 		)
 	}
 
+	// Clear FTS5 search index AFTER the transaction.
+	// ClearSearchIndex drops and recreates the contentless FTS5
+	// virtual table, which cannot run inside a transaction.
+	if err := l.db.ClearSearchIndex(); err != nil {
+		return fmt.Errorf(
+			"could not clear search index: %w", err,
+		)
+	}
+
 	l.logger.Info("all library tables cleared")
 
 	return nil