Fix: restore regen duration floor to 60s (was regressed to 20s)

Dropping to 20s caused MMAudio regen to request only 36s, which isn't
enough when open_clip downloads inside the GPU window on cold workers.
The 60s floor was the working value in 813c771 and needs to stay until
we confirm the snapshot_download pre-cache is actually effective.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -435,7 +435,7 @@ def _estimate_regen_duration(model_key: str, num_steps: int) -> int:
     """
     cfg  = MODEL_CONFIGS[model_key]
     secs = int(num_steps) * cfg["secs_per_step"] + cfg["load_overhead"]
-    result = min(GPU_DURATION_CAP, max(20, int(secs)))
+    result = min(GPU_DURATION_CAP, max(60, int(secs)))
     print(f"[duration] {cfg['label']} regen: 1 seg × {int(num_steps)} steps → {secs:.0f}s → capped {result}s")
     return result