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/queue/persistence.go

@@ -151,6 +151,7 @@ func (q *Queue) insertTrackBatch(tx *sql.Tx, batch []Track) error {
 		args = append(args, track.AudioFileID, track.Position)
 	}
 
+	// SAFETY: Multi-row INSERT with variable row count unsupported by sqlc. Placeholder count matches args length; no string interpolation.
 	query := "INSERT INTO queue_tracks (audio_file_id, position) VALUES " +
 		strings.Join(valuePlaceholders, ",")