fix(docker): use hf_hub_download for HuggingFace models to fix CDN 504s

urlretrieve against the HuggingFace CDN was consistently returning HTTP
504 in GitHub Actions runners for LaMa, NAFNet, and the OpenCV caffemodel.
The huggingface_hub library has built-in retry logic, resumable downloads,
and better CDN routing than bare urlretrieve.

- download_lama_model: urlretrieve → hf_hub_download (Carve/LaMa-ONNX)
- download_nafnet_model: urlretrieve → hf_hub_download (mikestealth/nafnet-models)
- download_opencv_colorize_models: caffemodel → hf_hub_download (space repo_type)
- _urlretrieve: retry count 3→5, flat 10s delay → exponential backoff (10/20/40/80s)
- Also reverts the SKIP_MODEL_DOWNLOADS=true from CI workflow (wrong approach)

.github/workflows/ci.yml

@@ -93,6 +93,5 @@ jobs:
           file: docker/Dockerfile
           push: false
           tags: ashim:ci
-          build-args: SKIP_MODEL_DOWNLOADS=true
           cache-from: type=gha,scope=unified
           cache-to: type=gha,mode=max,scope=unified