RealESRGAN's enhance() and rembg's remove() run in one opaque call, so the
progress bar froze at 30% for the whole inference. Add a time-based
heartbeat that advances the bar in a background thread while the model runs
and stops when it returns, so the bar moves instead of freezing. Verified
end to end on a GPU box (forced CPU): a 55s upscale emitted 26 steady ticks
then completed; background removal too.
Fixes#591
