ace-step: patch upstream infer-api + missing runtime deps + cache mount

Three upstream gaps surfaced once /generate was actually exercised:

  1. infer-api.py builds an 18-arg positional tuple but the pipeline
     expects 24 — first missing arg is `format`, so audio_duration
     shifts into format's slot and the pipeline calls len() on an
     int. Ship a patched copy of infer-api.py and COPY over upstream's
     in the Dockerfile. Also handle empty lora_name_or_path -> "none"
     (empty string trips HF Hub's repo-id validator).
  2. torchcodec + ffmpeg are required by the WAV save path but neither
     is in upstream requirements.txt. Without them every /generate
     runs to completion and then 500s at write-time.
  3. ACE-Step caches checkpoints at /root/.cache/ace-step/checkpoints
     (HARDCODED, not honored by HF_HOME). Mount our persistent dir
     there so the ~7 GB model survives container recreates.

Bench on A6000 (cached model, lo-fi hip hop, 60-step euler/apg):
  10s @ 27 steps -> 9.4s  (0.94x)
  30s @ 60 steps -> 11.2s (0.37x, ~2.7x realtime)
  60s @ 60 steps -> 14.8s (0.24x, ~4x realtime)

playbooks/deploy-ace-step.yaml

@@ -46,6 +46,12 @@ steps:
       dest: "{{ compose_dir }}/Dockerfile"
       mode: "0644"
 
+  - name: Upload patched infer-api.py (fixes upstream 18-arg tuple bug)
+    upload:
+      src: stacks/ace-step/infer-api.py
+      dest: "{{ compose_dir }}/infer-api.py"
+      mode: "0644"
+
   - name: Seed .env from template (only if absent)
     upload:
       src: stacks/ace-step/.env.example