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.

README.md

@@ -201,6 +201,22 @@ docker compose -f docker-compose.gpu.yml logs | grep -i sam
 
 If Docker created `./data/` as root and you can't write there without `sudo`, you can also use root's curl as above — the container reads the file regardless of owner.
 
+**Remove Background fails ("Install u2net or rembg")**
+
+The U2Net model auto-downloads (~176MB) from GitHub on first use, same as SAM. If that download fails (DNS/firewall, see above) and `rembg` isn't installed either, you'll see this error. Fix it the same way — download directly on the host:
+
+```bash
+mkdir -p ./data/models
+sudo curl -L -o ./data/models/u2net.onnx \
+  https://github.com/danielgatis/rembg/releases/download/v0.0.0/u2net.onnx
+```
+
+The file is ~176 MB. Once it exists at `./data/models/u2net.onnx`, the next "Remove Background" click picks it up — no rebuild or restart needed. Verify with:
+```bash
+docker compose logs -f | grep -i u2net
+# Should show: "U2Net model loaded successfully with OpenCV DNN"
+```
+
 **AI models not downloading (container DNS blocked)**
 
 If you ran `./install-local-gpu.sh`, this is already permanently fixed. Otherwise, the container's host firewall is blocking outbound DNS from the Docker bridge — apply the fix manually (does **not** affect container isolation):