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>

app/rembg_cli.py

@@ -5,26 +5,57 @@ the `rembg[cli]` extra (aiohttp, gradio, watchdog, ...) just to run a
 single background removal. Calling the `remove()` API directly from a
 tiny script keeps the image smaller while still giving pipeline.py a
 subprocess boundary to apply `nice` and a hard timeout to.
+
+Defaults target the event box (≈8GB RAM): u2net + alpha matting.
+`birefnet-general` (~928MB weights) OOMs here; override via REMBG_MODEL
+/ REMBG_ALPHA if the host has more headroom.
 """
 
 from __future__ import annotations
 
+import os
 import sys
+import traceback
 from pathlib import Path
 
 
+def _env_bool(name: str, default: bool) -> bool:
+    raw = os.environ.get(name)
+    if raw is None:
+        return default
+    return raw.strip().lower() in {"1", "true", "yes", "on"}
+
+
 def main() -> int:
     if len(sys.argv) != 3:
         print("usage: python -m app.rembg_cli <input> <output>", file=sys.stderr)
         return 2
 
-    from rembg import remove
+    # Keep model-download progress bars from polluting error parsing upstream.
+    os.environ.setdefault("TQDM_DISABLE", "1")
 
-    input_path = Path(sys.argv[1])
-    output_path = Path(sys.argv[2])
-    output_path.parent.mkdir(parents=True, exist_ok=True)
-    output_path.write_bytes(remove(input_path.read_bytes()))
-    return 0
+    try:
+        from rembg import new_session, remove
+
+        input_path = Path(sys.argv[1])
+        output_path = Path(sys.argv[2])
+        output_path.parent.mkdir(parents=True, exist_ok=True)
+
+        model = os.environ.get("REMBG_MODEL", "u2net")
+        alpha = _env_bool("REMBG_ALPHA", True)
+        session = new_session(model)
+        output_path.write_bytes(
+            remove(
+                input_path.read_bytes(),
+                session=session,
+                alpha_matting=alpha,
+            )
+        )
+        return 0
+    except Exception as exc:  # noqa: BLE001 - surface real cause to parent process
+        print(f"{type(exc).__name__}: {exc}", file=sys.stderr)
+        traceback.print_exc(file=sys.stderr)
+        return 1
 
 
 if __name__ == "__main__":