Auto-download U2Net model on startup; remove dead PyTorch U2Net module

U2Net only ever downloaded lazily on the first Remove Background click,
unlike SAM which retries on every container start. If that one attempt
failed (DNS/firewall) the model was never fetched again, surfacing as
"No background removal method available. Install u2net or rembg."

Mirrors the existing SAM auto-download/AUTO_DOWNLOAD_SAM pattern for
U2Net, and documents manual host-side recovery in the README.

Also deletes backend/app/services/u2net_model.py (hand-written
U2NET/U2NETP PyTorch classes) — unused since tools.py switched to
cv2.dnn.readNetFromONNX for background removal.

docker-compose.gpu.yml

@@ -120,6 +120,7 @@ services:
       - SECRET_KEY=${SECRET_KEY:-change-this-secret-key-in-production}
       - CORS_ORIGINS=*
       - AUTO_DOWNLOAD_SAM=${AUTO_DOWNLOAD_SAM:-true}
+      - AUTO_DOWNLOAD_U2NET=${AUTO_DOWNLOAD_U2NET:-true}
 
     # ── NVIDIA GPU passthrough ────────────────────────────────────────────────
     # Requires nvidia-container-toolkit; see prerequisites at top of this file.