Fix: backend resolves DiT model names to full paths

[Copilot]

The frontend sends model names (e.g. "acestep-v15-turbo-Q8_0") that were being passed directly to the dit-vae binary as file paths, causing failures. Additionally, ACESTEP_MODEL env vars pointing to non-existent paths (e.g. the .env.example placeholder /path/to/models/…) were trusted blindly, bypassing auto-detection.

Changes

• config/index.ts — resolveDitModel(): validates ACESTEP_MODEL path exists on disk before using it; falls through to MODELS_DIR auto-detection with a warning if not found

• services/acestep.ts — resolveParamDitModel(): new helper that owns the name→path resolution contract:

  1. No name → configured default
  2. Absolute path → pass through
  3. Exact <name>.gguf match in MODELS_DIR
  4. Prefix match with quant preference (Q8_0 → Q6_K → Q5_K_M → Q4_K_M → BF16)
  5. Nothing found → configured default

  Used in both spawn mode and HTTP mode — the UI sends a name, the backend resolves the path.

// Before — name used directly as a file path
const ditModel = params.ditModel ? params.ditModel : config.acestep.ditModel;

// After — backend owns path resolution
const ditModel = resolveParamDitModel(params.ditModel);

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