Add BEN2 and BiRefNet-HR as selectable Remove Background models

BEN2 becomes the new default local backend (clean cutouts, strong on
hair/fur edges), with BiRefNet-HR available as a high-res/print
alternate and U2Net kept as the lightweight fallback. Both are
MIT-licensed and download weights from HuggingFace on first use
(cached via the existing hf_cache bind mount), unlike U2Net/SAM which
need an explicit download script.

- config: new BG_REMOVAL_MODEL setting (default "ben2")
- tools.py: remove-background-base64 now tries local backends in
  order (request.model override > BG_REMOVAL_MODEL > ben2/u2net),
  falling back to rembg's birefnet-general session as a last resort
- requirements.gpu.txt / Dockerfile.gpu: add ben2 + transformers deps
  needed for the new backends, with a build-time smoke test for ben2
- frontend: model dropdown in the Remove Background dialog, threaded
  through api.js to the new request field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ro4PwQKvSc3CH19LSN21Ht

Dockerfile.gpu

@@ -54,6 +54,10 @@ RUN echo "BUILDID=$BUILDID" && pip install --no-cache-dir -r requirements.gpu.tx
 RUN python -c "from rembg import remove; print('rembg OK')" \
     || echo "WARNING: rembg unavailable — Remove Background disabled"
 
+# Smoke-test ben2 (weights download from HuggingFace on first use)
+RUN python -c "import ben2; print('ben2 OK')" \
+    || echo "WARNING: ben2 unavailable — Remove Background falls back to U2Net/rembg"
+
 # Copy backend application
 COPY backend/ .