Bria's /remove_background endpoint accepts background_color='Transparent'
when paired with an alpha-capable container/codec; pass output_container_and_codec=
'webm_vp9' in that case (otherwise mp4_h264, unchanged). The returned WebM
flows through ComfyUI's VIDEO type as a file reference, so Save Video with the
default auto/auto settings stream-copies it byte-for-byte with the alpha plane
intact (covered by new test_save_to_auto_preserves_vp9_alpha_via_stream_copy).
BriaTransparentVideoBackground is kept as-is because it remains the supported
path for the IMAGE+MASK workflow: Get Video Components currently cannot expose
alpha for VP9 streams (PyAV's default vp9 decoder drops the side alpha layer),
so the per-frame compositing workflow still needs the dedicated node that
decodes via libvpx-vp9 itself.
