Fix acestep-cpp model path resolution and cover mode spawn pipeline

[Copilot]

MODELS_DIR=./models (and sibling relative paths) from .env were resolved against the server's CWD (<repo>/server/) instead of the project root, causing all model lookups to return empty strings and every generation to fail with LM model not found — run models.sh first even when models were present. Cover/audio-to-audio mode also incorrectly ran ace-qwen3 before dit-vae, despite dit-vae deriving audio codes from the source audio directly.

Path resolution (server/src/config/index.ts)

Added resolveFromRoot(p) — resolves relative paths against the absolute APP_ROOT (from __dirname) instead of leaving them as raw strings for Node's CWD to misinterpret. Applied to all nine path env vars:

MODELS_DIR, DATABASE_PATH, AUDIO_DIR,
ACE_QWEN3_BIN, DIT_VAE_BIN,
ACESTEP_MODEL, LM_MODEL, TEXT_ENCODER_MODEL, VAE_MODEL

Cover mode (server/src/services/acestep.ts)

• useSpawnMode(params?) — cover/passthrough jobs (sourceAudioUrl or audioCodes) now only require ditVaeBin; lmBin is only required when the LM step is actually needed (text-to-music)
• runViaSpawn — skip ace-qwen3 when sourceAudioUrl is set; request.json is passed directly to dit-vae (same pattern as existing audioCodes passthrough)
• runViaSpawn — audio_cover_strength added to the spawn-mode request JSON
• processGeneration — passes params to useSpawnMode() so cover-mode jobs enter spawn mode when only ditVaeBin is available

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