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

backend/app/services/local_diffusion.py

@@ -11,8 +11,8 @@ Supported model families:
   sd2x  → StableDiffusion2*Pipeline               (SD 2.x)
   sd15  → StableDiffusionPipeline                 (SD 1.5)
 
-Requires: diffusers>=0.29.0, transformers, accelerate, safetensors
-          (all in requirements.gpu.txt)
+Requires: diffusers>=0.28.0,<0.29.0, transformers, accelerate, safetensors
+          (all in requirements.gpu.txt — pinned <0.29.0 for PyTorch 2.1.x compatibility)
 """
 from __future__ import annotations