Fix cover/repaint generation failures: wrong audio dir, bad request JSON format, missing debug logs

[Copilot]

Cover mode and regular generations were silently failing due to a combination of a wrong AUDIO_DIR path, LM-specific fields leaking into dit-vae request JSONs, and overly strict route validation. Debug logging was absent, making these failures nearly impossible to diagnose.

Root causes & fixes

server/src/config/index.ts — Wrong audioDir (primary failure cause)

config.storage.audioDir resolved to <project>/public/audio (APP_ROOT = repo root), but:

• LocalStorageProvider writes to server/public/audio
• Express serves /audio/ from server/public/audio

resolveAudioPath('/audio/reference-tracks/…') was looking in the wrong directory — --src-audio pointed at a nonexistent path, crashing dit-vae.

// Before — wrong: resolves to <project>/public/audio
audioDir: resolveFromRoot(process.env.AUDIO_DIR || path.join(APP_ROOT, 'public', 'audio'))

// After — correct: resolves to <project>/server/public/audio
const SERVER_ROOT = path.resolve(__dirname, '../..');
audioDir: resolveFromRoot(process.env.AUDIO_DIR || path.join(SERVER_ROOT, 'public', 'audio'))

server/src/services/acestep.ts — Mode-specific request JSON

All modes were receiving the same JSON including LM-specific fields (lm_temperature, lm_cfg_scale, vocal_language, etc.) that only ace-qwen3 needs. audio_cover_strength was also written into text2music requests.

Now each mode gets only its relevant fields:

• text2music → LM fields for ace-qwen3
• cover → audio_cover_strength
• repaint → repainting_start, repainting_end
• passthrough → audio_codes; throws early if missing

Mode detection is now driven by explicit taskType flags (isCover, isRepaint, isPassthru, skipLm) rather than ad-hoc sourceAudioUrl presence checks.

server/src/routes/generate.ts — Validation too strict for cover/repaint

Custom-mode requests with only sourceAudioUrl (no style/lyrics) were rejected. Added requiresSourceAudio exception so cover/audio2audio/repaint requests pass through.

Debug logging added

Every generation now emits structured logs covering:

• API entry: taskType, ditModel, sourceAudio URL, repaint region, user ID
• Spawn: full ace-qwen3 and dit-vae CLI commands (previously truncated to 6 args), request JSON written to disk, resolved model paths
• resolveAudioPath: every URL→filesystem resolution
• Errors: validation failures and binary errors always include the full message

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