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/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