Upgrade a legacy Consent runtime to v2 storage

Migrates eligible legacy runtime configuration to shared v2 storage and schedules the existing historical backfill. Shared entitlement and storage checks remain authoritative. completed means the lifecycle command completed, not that the historical backfill finished. Supply the runtime ID and version from lifecycle metadata; stale values return CONFLICT. 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.