Undo scheduled cancellation

Restores renewal when cancellation/expiry is scheduled for the explicit product. Enterprise remains sales-managed. Reuse requestId for retries. Requires billing.write, a verified person and owner role. Subject to rollout-agent-access-writes (default off) and allow-public-agent-writes before account/provider effects. Return and person handoff URLs always use the canonical Dashboard origin; redirect URLs, payment details and provider tokens are not accepted.