Get Checkpoint Download Urls

Get presigned download URLs for checkpoint restoration.

Used when restoring a checkpoint that doesn't exist locally (e.g., after cloning a repo on a new machine). Returns URLs for both the HEAD bundle (if the checkpoint was based on a HEAD commit) and the checkpoint bundle.

Supports two checkpoint formats:

1. Regular checkpoints: Returns HEAD bundle URL + checkpoint bundle URL
2. Base checkpoints (base:{sha}): Returns only base_ref bundle URL as checkpoint_bundle_url

The checkpoint metadata is looked up from the database (stored when the checkpoint was originally synced). If head_sha is present, both bundles are needed; otherwise only the checkpoint bundle.