Check Slack app creation status

Report whether Slack has sent an events-URL verification challenge to a surface's Slack webhook. Slack issues that challenge when an app is created from a manifest, so a verifiedAt newer than the one observed before the manifest was handed out is evidence the customer's Slack app now exists. Verification markers expire an hour after they are recorded, and the check is advisory: the OAuth install remains the authority on whether an integration is usable.