fix(zerogpu-video): replace opencv export_to_video with PIL + ffmpeg

diffusers export_to_video fails with 'export_to_video requires the OpenCV
library but it was not found in your environment'. Replacing with PIL PNG
frame dump + standard system ffmpeg subprocess, avoiding missing opencv
dependency on ZeroGPU container.

app.py

@@ -1271,7 +1271,6 @@ def generate_zerogpu_video(
     try:
         import torch
         from diffusers import DiffusionPipeline, DPMSolverMultistepScheduler
-        from diffusers.utils import export_to_video
 
         device = "cuda" if torch.cuda.is_available() else "cpu"
         dtype = torch.float16 if device == "cuda" else torch.float32
@@ -1303,7 +1302,27 @@ def generate_zerogpu_video(
             generator=generator
         ).frames[0]
 
-        export_to_video(video_frames, out_video_path, fps=int(fps))
+        import numpy as np
+        from PIL import Image
+
+        ffmpeg_bin = shutil.which("ffmpeg") or "/usr/bin/ffmpeg"
+        with tempfile.TemporaryDirectory() as tmpdir:
+            for idx, frame in enumerate(video_frames):
+                if isinstance(frame, np.ndarray):
+                    img = Image.fromarray(frame)
+                else:
+                    img = frame
+                img.save(os.path.join(tmpdir, f"frame_{idx:05d}.png"))
+            subprocess.run(
+                [
+                    ffmpeg_bin, "-y", "-framerate", str(int(fps)),
+                    "-i", os.path.join(tmpdir, "frame_%05d.png"),
+                    "-c:v", "libx264", "-pix_fmt", "yuv420p",
+                    out_video_path
+                ],
+                check=True,
+                capture_output=True
+            )
         return out_video_path
     except Exception as exc:
         print(f"[ZeroGPU Video] Direct pipeline exception: {exc}, running ffmpeg dynamic visualizer fallback...", flush=True)