feat(forum): The League — Vera's forum on the record, swept, desked, announced (Fable) (#295)

PR-062 end to end: the vision doc, migration 0058 (rooms/topics/posts/flags/members + the append-only forum_events ledger), the Haiku sweep with a stronger-model second look (reversible verbs only), the public room at /forum with guidelines and a public moderation log, the solo-operator Forum Desk, and the launch companions (The League Is Open dispatch, the struck OG seal, Margin wayfinding, the seeding kit).