Disconnect Slack

Mark all of the user's installations revoked.

Tokens stay encrypted at rest until the user reinstalls (which overwrites them); Slack-side revocation is the user's uninstall in Slack, which the Events API webhook also records.

Any recorded revocation reason goes with them: the user has now asked for this state, so the settings page must stop attributing it to an earlier takeover. Rows are walked rather than bulk-updated because the reason lives inside config, and reassigning it is what makes SQLAlchemy notice the JSON column changed.