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>

backend/app/services/render_blender.py

@@ -605,6 +605,7 @@ def render_cinematic_to_file(
 
     env = dict(os.environ)
     env["EGL_PLATFORM"] = "surfaceless"
+    env["PYTHONUNBUFFERED"] = "1"  # Force unbuffered stdout for live frame progress
 
     glb_arg = "" if use_usd else str(glb_path)
     cmd = [