Promote checkpoint to snapshot #3809
Description
A checkpoint can be promoted to a full snapshot that enters the standard distribution lifecycle.
The user provides the checkpoint ID and a name for the new snapshot. Only standard snapshot params are populated on the snapshot entity (cpu, mem, disk, gpu, entrypoint). No extra params like env, labels, or network settings are inherited -- a snapshot is a snapshot, a checkpoint is a checkpoint.
The snapshot is created with the checkpoint's ref and a reference to a runner that already has the image, so it transitions quickly to ACTIVE and gets distributed proactively.