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>

.env.example

@@ -13,9 +13,21 @@ DATA_HOST_DIR=./data
 # Spec asked for 121212 which exceeds TCP max (65535). Use 12121.
 SFTP_HOST_PORT=12121
 
+# Minutes until SFTPGo closes an idle connection. Android/SSHJ clients
+# often sit idle after upload; the default 15m shows up as a spurious EOF.
+SFTPGO_IDLE_TIMEOUT=120
+
 # --- Pipeline tuning (see make_random.py for background) -----------------
 OUTPUT_COUNT=3
+# Remix-form default / legacy; auto variants use the min/max range below.
 BLEND_OPACITY=30%
+BLEND_OPACITY_MIN=10
+BLEND_OPACITY_MAX=50
 FILTER_TIMEOUT=120
+FILTER_TIMEOUT_LONG=300
 MAX_FILTER_ATTEMPTS=8
+MAX_EDGE_PX=2000
+# birefnet-general OOMs on the ~8GB box; u2net + alpha ≈ `rembg i -a`
+REMBG_MODEL=u2net
+REMBG_ALPHA=1
 NICE_LEVEL=18