fix: unbuffered stdout for live cinematic frame progress

Two fixes for frame progress not appearing in frontend:
1. Added flush=True to all print() calls in cinematic_render.py
2. Set PYTHONUNBUFFERED=1 in subprocess environment

Without these, Python buffers stdout inside Blender, so all frame
progress lines arrive in a batch after the process exits instead
of streaming line-by-line during rendering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

render-worker/scripts/cinematic_render.py

@@ -849,10 +849,10 @@ def main():
         bpy.ops.render.render(write_still=True)
         elapsed = _time.time() - _render_start
         fps_so_far = frame / elapsed
-        print(f"[cinematic_render] Frame {frame}/{frame_count} -- {elapsed:.1f}s elapsed ({fps_so_far:.2f} fps)")
+        print(f"[cinematic_render] Frame {frame}/{frame_count} -- {elapsed:.1f}s elapsed ({fps_so_far:.2f} fps)", flush=True)
 
     total = _time.time() - _render_start
-    print(f"[cinematic_render] Cinematic render complete: {frame_count} frames in {total:.1f}s ({frame_count/total:.2f} fps avg)")
+    print(f"[cinematic_render] Cinematic render complete: {frame_count} frames in {total:.1f}s ({frame_count/total:.2f} fps avg)", flush=True)
 
 
 if __name__ == "__main__":