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>

Dockerfile

@@ -25,6 +25,7 @@ ARG GMIC_DEB_URL=https://gmic.eu/get_file.php?file=linux/gmic_4.0.2_debian12_boo
 RUN apt-get update && apt-get install -y --no-install-recommends \
         ca-certificates \
         curl \
+        imagemagick \
         python3 \
         python3-pip \
     && curl -fsSL "$GMIC_DEB_URL" -o /tmp/gmic.deb \
@@ -49,9 +50,10 @@ RUN pip install --break-system-packages -r requirements.txt
 
 COPY app ./app
 
-# rembg downloads its ONNX model (u2net, ~176MB) from GitHub on first use
+# rembg downloads its ONNX model (default u2net) from GitHub on first use
 # and caches it under $HOME/.u2net — persisted via the `rembg_cache`
-# volume in compose.yml so it survives container restarts.
+# volume in compose.yml so it survives container restarts. Alpha matting
+# is enabled by default (REMBG_ALPHA=1).
 
 # Overridden per-service in compose.yml (`worker` -> python -m app.worker,
 # `web` -> uvicorn app.main:app).