docs(06-03): add SAFETY comments to all 12 hand-crafted SQL statements

- 7 SAFETY comments in search.go (FTS5 MATCH/INSERT/DELETE operations)
- 3 SAFETY comments in library.go (FTS5 INSERT/DELETE in commitNewAudioFile, updateAudioFileMetadata)
- 1 SAFETY comment in rescan.go (FTS5 DELETE in clearAllLibraryData)
- 1 SAFETY comment in persistence.go (variable-count multi-row INSERT)
- Cross-references link library.go/rescan.go back to search.go
- Two-part format: why sqlc can't handle it + what makes it safe

backend/library/rescan.go

@@ -161,6 +161,7 @@ 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 {