Keep Slack channel authorization in sync

[16francej]

Summary

• Persist verified Slack channel metadata and membership before admitting a turn, using the existing directory store.
• Publish fresh channel listings instead of replaying cached snapshots; keep expensive roster reads throttled.
• Preserve snapshot ordering and existing broker revocation checks. No new capability authority.

Verification

• 326 affected tests, PostgreSQL ordering/migration checks, and static gates passed.
• Live end-to-end replays pass for delayed channel events and stale private rosters; an existing token is denied after private-channel removal.
• Independent admission/security and concurrency reviews completed.