Add APE+CUE splitting via ffmpeg convert-then-split

[mprachar]

Summary

• Adds APE (Monkey's Audio) + CUE sheet splitting support alongside existing FLAC+CUE
• APE files are converted to a temporary FLAC via ffmpeg (compression level 0), then split using the existing splitFLAC() pipeline — zero changes to the proven FLAC splitting path
• Graceful fallback: when ffmpeg is not installed, returns ErrFFmpegNotFound instead of silently failing
• resolveCueAudioPath() now tries .ape alongside .flac in all fallback paths

Motivation

Closes Unpackerr/unpackerr#605 — APE+CUE albums are common in music downloads but were silently ignored by the .flac-only format gate. Lidarr cannot import single-image APE files, so they sit in completed directories forever.

Approach: Convert-Then-Split (vs alternatives)

A pure-Go APE codec doesn't exist (and a prior attempt was described as "pretty bleak"). This PR takes the minimal-risk approach:

1. Detect .ape audio file via CUE sheet resolution
2. Convert APE → temporary FLAC via ffmpeg -c:a flac -compression_level 0
3. Split using the existing splitFLAC() — sample-accurate, battle-tested
4. Cleanup temp FLAC after splitting

The conversion is lossless (APE → PCM → FLAC is a bit-for-bit round-trip). Compression level 0 minimizes conversion time since the file is split immediately anyway.

Changes

File	Change
cue.go	Format dispatch: .flac → splitFLAC(), .ape → convertAndSplitAPE()
cue.go	New convertToFLAC() — ffmpeg shell-out with temp dir cleanup
cue.go	resolveCueAudioPath() — adds .ape to all fallback resolution paths
errors.go	New ErrFFmpegNotFound; updated ErrUnsupportedAudio message
cue_test.go	4 new tests (all skip gracefully when ffmpeg unavailable)

New tests

• TestCueAPEConvertAndSplit — full end-to-end: generate FLAC → convert to APE → split via CUE → verify FLAC tracks + VorbisComment tags
• TestCueAPENoFFmpeg — verifies ErrFFmpegNotFound when PATH is overridden
• TestCueWavReferenceAPEFile — CUE says .wav but only .ape exists
• TestCueBaseNameAPEFallback — CUE filename mismatch, resolved via basename + .ape

Test plan

• go test ./... passes (existing FLAC tests unaffected)
• APE tests pass when ffmpeg is installed
• APE tests skip gracefully when ffmpeg is not installed
• Manual test with real APE+CUE album on Unpackerr instance

Prerequisites

Systems using APE splitting need ffmpeg installed with APE decoder support:

ffmpeg -decoders 2>/dev/null | grep -i ape
# Expected: " A....D ape    Monkey's Audio"

🤖 Generated with Claude Code

[davidnewhall]

Thanks for putting time into this. We need to remove the call to ffmpeg before merging. Might be a while since no one is making an APE library for Go.

[mprachar]

I see, is there a problem with that dependency that doesn’t affect ‘lil ol me using it at home? It’s been working great. Best Regards, Mike

[davidnewhall]

This module is used by others. I do not wish to add calls to os.Exec as those calls can get flagged by security scanning tools. If someone swapped the ffmpeg binary for something else, it could raise privileges. Indeed, these are problems home users do not typically encounter.