Pre-deployment checks for a serving spec (platform admin)

Read-only. Evaluates everything that must already be true for a deployment with the given spec to admit, place, and register: the catalog row is seeded with pricing, the weights ref is published, the GPU footprint fits the serving capacity budget, prepared nodes exist, and readiness probing is configured. Each check is only applied when its query parameters are present, so the UI can preflight incrementally as a form is filled in.