Update the Consent runtime backend

Applies the available backend version and database migrations using shared role, billing and update-attempt rules. Read lifecycle metadata first and send its current runtime ID and backend version. A stale target returns CONFLICT before mutation. After an uncertain response, read lifecycle metadata before retrying. Requires consent.lifecycle.write, projects.read and projects.write, with an admin or owner role for update/upgrade, or an owner role for delete. Subject to rollout-agent-access-writes and allow-public-agent-writes. Organization API keys cannot call this operation.