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

backend/app/config.py

@@ -46,6 +46,11 @@ class Settings(BaseSettings):
     # Allow per-edit model override
     allow_model_override: bool = True
 
+    # Remove Background — preferred local model when request.model="auto"
+    # Options: ben2 (default, best for clean cutouts/hair), birefnet-hr (best
+    # for high-res/print work), u2net (lightweight, smallest download)
+    bg_removal_model: str = "ben2"
+
     # Local GPU diffusion (AI_PROVIDER=local_gpu)
     auto_download_models: bool = True      # download HF models on first use
     local_gpu_max_pipelines: int = 2       # max diffusion pipelines kept in GPU memory