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>

.gitignore

@@ -4,6 +4,13 @@
 # Runtime data (bind-mounted, never belongs in git)
 /data/
 
+# Local rembg/withoutbg comparison scratch
+/compare-bg/inputs/
+/compare-bg/outputs/
+/compare-bg/model-cache/
+/compare-bg/compare.log
+.venv-compare/
+
 # Python
 __pycache__/
 *.pyc