Add FLAC (.flac) file support

[akrbout]

What

Adds support for converting .flac files from Serato crates to rekordbox XML.

Why

FLAC is a popular format for DJs who care about audio quality, but the tool previously skipped all .flac files with Unsupported format: .flac. In my library that was ~1300 tracks out of ~4200.

How

• New extract_flac.py module using mutagen.flac.FLAC
• Serato stores markers in FLAC as Vorbis comments (SERATO_MARKERS_V2, SERATO_BEATGRID) — the payload format is identical to M4A (double base64 with application/octet-stream wrapper)
• Reuses extract_m4a.parse_serato_hot_cues() for hot cues (same double-base64 format)
• Reuses extract_mp3.parse_beatgrid_markers() for beatgrid (same binary structure after base64 decode)
• Reads BPM from BPM/TBPM Vorbis comments, key from INITIALKEY
• Updated serato2rekordbox.py: added .flac to format dispatcher and Kind mapping (FLAC File)

Testing

Tested on my Serato library (~4200 tracks, ~1300 FLAC):

• Before: 2898 tracks converted, ~1300 FLAC files skipped as unsupported
• After: 4198 tracks converted, 0 FLAC-related failures
• Hot cues, beatgrids, BPM, and key all correctly extracted from FLAC files
• A handful of corrupt/invalid FLAC files are gracefully handled as errors (same as corrupt MP3/M4A would be)

Files changed

• extract_flac.py (new, 164 lines)
• serato2rekordbox.py (+8 lines: import, format dispatch, Kind mapping)