Best-effort lookup of an activating member's email in the Slack
workspace. On a hit, flips slackInvited:true and stamps slackInvitedAt
without sending a fresh invite. Races against a 3s timeout and swallows
all errors so activation never blocks on Slack.
- Promotes SlackService.findUserByEmail from private to public so the
helper can call it without a wrapper.
- New activity-log action: slack_access_auto_detected (actor = subject).
- Idempotent: short-circuits when slackInvited is already true.
Callers wired in next commit.
