fix(spaces): _duration_for must accept progress kwarg to match _execute_workflow

ZeroGPU calls the duration estimator with the same args it'll pass to the
@spaces.GPU function. After adding progress as the 7th arg to
_execute_workflow in 27ffbbc, _duration_for raised TypeError on every
generation. Add progress to its signature; we ignore it for estimation.

backend.py

@@ -99,9 +99,14 @@ def _duration_for(
     mode: str,
     preset: str,
     multiplier: float = 1.0,
+    progress: Any = None,
 ) -> int:
     """ZeroGPU duration estimator. Same signature as _execute_workflow.
 
+    `progress` is a gr.Progress instance forwarded by the caller; we ignore it
+    here (estimator doesn't emit progress) but must accept it positionally so
+    ZeroGPU can call us with the same arg list it'll use for _execute_workflow.
+
     Estimate = (base × preset multiplier + cold-cache buffer + per-frame VAE
     decode time) × retry multiplier, clamped to [60s, 900s]. The 900s ceiling
     keeps a single failed call from torching the daily quota.