sao: actually pass cfg_scale through to the pipeline

server.py accepted cfg_scale in the request schema and the README
documented its 0–20 range, but the pipeline call never received it
— so changing cfg_scale between requests silently produced identical
output (the pipeline ran at its own default of 7.0 every time). Add
guidance_scale=req.cfg_scale to the pipe(...) call.

Verified: (prompt, seed, steps) held constant, cfg_scale=3.0 vs 15.0
now produce different SHA256s; same triple at cfg_scale=7.0 is
deterministic across repeated calls.

stacks/stable-audio-open/server.py

@@ -70,6 +70,7 @@ def sfx(req: SfxRequest):
         num_inference_steps=req.steps,
         audio_end_in_s=req.duration,
         num_waveforms_per_prompt=1,
+        guidance_scale=req.cfg_scale,
         generator=generator,
     ).audios