Add 12 tests for background-removal downscaling (resize gate, portrait
orientation, mask upscale) and OOM model fallback (retry with u2net,
progress callback, no-retry guards, cascading failure).
Add OOM propagation tests to face-detection, noise-removal,
red-eye-removal, and OCR -- the four AI features that were missing them.
Skip alpha matting on CPU (pymatting's sparse matrices are the main
memory hog), auto-downscale images above 2048px before sending to
rembg, and retry with the lighter u2net model when OOM is detected.