feat(15-01): migrate FTS5 search_index to contentless_delete=1

- Add contentless_delete=1 to search_index.sql schema file
- Update ClearSearchIndex CREATE statement to match schema
- Replace DeleteSearchIndex no-op with real DELETE WHERE rowid
- Add migration 8: drop/recreate/repopulate FTS5 table
- Update library.go comment about FTS entry lifecycle

backend/database/sql/schemas/search_index.sql

@@ -4,5 +4,6 @@ CREATE VIRTUAL TABLE IF NOT EXISTS search_index USING fts5(
     artist,
     album,
     content='',
+    contentless_delete=1,
     tokenize='unicode61 remove_diacritics 2'
 );