Retract a fact

Marks the fact retracted and writes the audit trail. Two cascades follow: facts DERIVED from it are retracted too (cascadedFactIds), and facts this one had SUPERSEDED come back to active (revivedFactIds) unless they were separately retracted on their own merits. Deliberately NOT gated by FACTS_API_ENABLED — the read routes above are, but a tenant must always be able to remove a fact. brain:write is the floor; FactsService elevates the requirement to brain:admin for billing_event / human_declared / legal-source facts once it has read the row (403 from there, not from the guard). For erasure rather than retraction, see the GDPR cascade on the admin surface (docs/api.md).

Required scope: brain:write.