Fix build cache, DNS/model download, and AI Edit error handling

Build / pip layer fixes:
- Add BUILDID ARG to Dockerfile.gpu; pass from docker-compose.gpu.yml build args
  so pip layers can be force-busted without --no-cache:
    BUILDID=$(date +%s) docker compose -f docker-compose.gpu.yml up --build

Model download (DNS-blocked environments):
- Change HF model cache from named volume to ./data/hf_cache bind mount
  so models can be pre-downloaded on the host (no rebuild needed)
- Remove now-unused hf_model_cache named volume
- README: add iptables fix + huggingface-cli offline download instructions

Error handling improvements:
- ai_edit_region: catch ConnectError/Errno-3 → return 503 with exact fix commands
- _require_remote: give actionable message when local_gpu provider fails to load
- _build_provider: catch AttributeError (torch.xpu from wrong diffusers) not just ImportError
- local_diffusion.py: fix docstring to reflect <0.29.0 pin

https://claude.ai/code/session_01WVDg7amsy1TTtxvpku7bcM

Dockerfile.gpu

@@ -42,10 +42,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Python dependencies — base + GPU extras
+# BUILDID forces pip layers to re-run when you need fresh packages without a full --no-cache:
+#   BUILDID=$(date +%s) docker compose -f docker-compose.gpu.yml up --build
+ARG BUILDID=1
 COPY backend/requirements.txt .
 COPY backend/requirements.gpu.txt .
-RUN pip install --no-cache-dir -r requirements.txt
-RUN pip install --no-cache-dir -r requirements.gpu.txt
+RUN echo "BUILDID=$BUILDID" && pip install --no-cache-dir -r requirements.txt
+RUN echo "BUILDID=$BUILDID" && pip install --no-cache-dir -r requirements.gpu.txt
 
 # Smoke-test rembg (model downloads on first use)
 RUN python -c "from rembg import remove; print('rembg OK')" \