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/database/search.go

@@ -117,24 +117,41 @@ func (d *DB) InsertSearchIndex(
 	return err
 }
 
-// DeleteSearchIndex removes a row from the FTS5 search_index.
-func (d *DB) DeleteSearchIndex(rowid int64) error {
-	// SAFETY: FTS5 virtual table DELETE unsupported by sqlc. Rowid is parameterized.
-	_, err := d.db.ExecContext(d.Ctx, `
-		DELETE FROM search_index WHERE rowid = ?
-	`, rowid)
-
-	return err
+// DeleteSearchIndex is a no-op for contentless FTS5 tables.
+// Contentless FTS5 (content=”) does not support DELETE.
+// Stale entries are harmless: they point to rowids that no longer
+// match in track_metadata, so JOINs in search queries filter them
+// out.  The index is fully rebuilt during FullRescan.
+func (d *DB) DeleteSearchIndex(_ int64) error {
+	return nil
 }
 
 // ClearSearchIndex removes all rows from the FTS5 search_index.
+// The search_index is a contentless FTS5 table (content=”), which
+// does not support DELETE.  We drop and recreate it instead.
 func (d *DB) ClearSearchIndex() error {
-	// SAFETY: FTS5 virtual table DELETE unsupported by sqlc. No parameters; unconditional delete.
-	_, err := d.db.ExecContext(d.Ctx, `
-		DELETE FROM search_index
-	`)
+	// SAFETY: FTS5 contentless table cannot be DELETEd from.
+	// Drop + recreate is the only way to clear it.  No parameters.
+	if _, err := d.db.ExecContext(d.Ctx,
+		`DROP TABLE IF EXISTS search_index`,
+	); err != nil {
+		return fmt.Errorf("could not drop search_index: %w", err)
+	}
 
-	return err
+	if _, err := d.db.ExecContext(d.Ctx, `
+		CREATE VIRTUAL TABLE IF NOT EXISTS search_index USING fts5(
+			file_path,
+			title,
+			artist,
+			album,
+			content='',
+			tokenize='unicode61 remove_diacritics 2'
+		)
+	`); err != nil {
+		return fmt.Errorf("could not recreate search_index: %w", err)
+	}
+
+	return nil
 }
 
 // RebuildSearchIndex repopulates the FTS5 search_index from