refactor: replace Git LFS with runtime model download from Hugging Face

Remove ~170MB of model files from the repository (LFS + committed).
Models are now downloaded at runtime from Hugging Face on first use
and cached to the OS cache directory with progress reporting and retries.

- Add internal/models/download.go: runtime downloader with retry, progress bar, checksums
- Remove go:embed for ONNX models (keep tokenizers embedded)
- Use file-based ONNX session loading instead of byte-slice
- Add scripts/download-models.sh for dev/CI model setup
- Update Makefile with setup-models target
- Update workflow-prepare.sh to download models in CI
- Set lfs: false in all CI workflows
- SHA256: bge=828e14..., cross-encoder=5d3e70...

.github/workflows/release.yaml

@@ -25,7 +25,7 @@ jobs:
             node-cache-dependency-path: "ui/package-lock.json"
             node-output-path: "ui/dist"
             node-embed-path: "internal/worker/static"
-            lfs: true
+            lfs: false
         secrets: inherit
 
     commit-marketplace: