feat(20-01): implement OGG Vorbis tag writer with custom page parser and CRC32

- Custom OGG page parser/writer with MSB-first CRC32 lookup table (ogg.go)
- Vorbis Comment packet parse/serialize with raw byte preservation (ogg_vorbis.go)
- METADATA_BLOCK_PICTURE base64 encoding for cover art with legacy field stripping
- Multi-stream and non-Vorbis OGG rejection with clear error messages
- Pipeline integration: FormatOGG constant, .ogg in DetectFormat, writeOggTags dispatch
- Lenient-read/strict-write: warn on CRC mismatch, always write correct CRCs
- Page sequence renumbering and crash-safe writes via AtomicWrite

backend/tagwriter/pipeline.go

@@ -146,6 +146,8 @@ func (tw *TagWriter) WriteTrackTags(trackID int64, changes TagChanges) error {
 		err = writeFlacTags(tw.logger, audioFile.FilePath, changes)
 	case FormatWAV:
 		err = writeWavTags(tw.logger, audioFile.FilePath, changes)
+	case FormatOGG:
+		err = writeOggTags(tw.logger, audioFile.FilePath, changes)
 	default:
 		err = fmt.Errorf("%w: %s", errUnsupportedFormat, format)
 	}