Fix COVER mode MP3 failure: AUDIO_DIR path mismatch breaks file resolution for dit-vae

[Copilot]

.env.example set AUDIO_DIR=./public/audio, which resolveFromRoot expands against APP_ROOT (project root) → <root>/public/audio. But LocalStorageProvider hardcoded its path three levels up from server/src/services/storage/ → <root>/server/public/audio. Files were written to one place, binary told to read from another — drmp3_open_file_and_read_pcm_frames_f32 opened nothing and returned NULL.

Changes

• .env.example: AUDIO_DIR=./public/audio → ./server/public/audio (correct relative path from APP_ROOT)
• storage/local.ts: Replaced hardcoded path.join(__dirname, '../../../public/audio') with config.storage.audioDir — storage provider now always writes where the rest of the system reads
• server/src/index.ts: Express /audio static mount now uses config.storage.audioDir instead of hardcoded path.join(__dirname, '../public/audio') — all three consumers share one value
• config/index.ts: Comment updated to reflect single-source-of-truth design
• acestep.ts: Reverted incorrect ensureWavFormat FFmpeg shim — audiohacking/acestep.cpp@copilot/add-wav-mp3-conversion decodes MP3 natively via dr_mp3.h