Moderation and authorization for the forum #22

Outcome

Forum reads and writes respect the repository authorization model, with operator moderation tools.

Work

• Define who may read and post; align with the readable_by predicates used elsewhere.
• Operator surfaces: lock topic, hide post, ban actor — each recorded as an audit event.
• Rate limiting ported from the Effect policy.

Acceptance criteria

• A viewer without access sees no private content anywhere on the forum surface.
• Every moderation action produces a durable audit event.

Verification

Authority and audit tests; mix precommit.