feat: harden pipeline UX — preprocess, rembg alpha, remix/lightbox

Downscale to 2000px JPEG before rembg, random blend opacity, timeout
retries, per-variant remix prefill, lightbox, and longer SFTP idle.

Co-authored-by: Cursor <cursoragent@cursor.com>

compare-bg/README.md

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+# Background-removal comparison
+
+Inbox originals from `live.f12.rocks` vs. backends.
+
+## Layout
+
+- `inputs/` — originals from boka inbox
+- `outputs/` — results, naming:
+
+```
+<stem>__rembg-u2net__no-alpha.png
+<stem>__rembg-u2net__alpha.png
+<stem>__rembg-default__no-alpha.png
+<stem>__rembg-default__alpha.png
+<stem>__rembg-birefnet-general__no-alpha.png
+<stem>__rembg-birefnet-general__alpha.png
+<stem>__withoutbg-open-weights.png
+```
+
+- `rembg-default` = `rembg.remove()` without session (library default = u2net)
+- `rembg-u2net` = explicit `new_session("u2net")` — expect identical to default
+- `withoutbg` open-weights includes matting in-graph (~2GB RAM claimed)
+
+## Re-run
+
+```bash
+./run.sh
+```
+
+Skips existing non-empty outputs. Models cached in `model-cache/`.