feat(slack): autoFlagPreExistingSlackAccess helper

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.

server/utils/slack.ts

@@ -104,7 +104,7 @@ export class SlackService {
   /**
    * Find user in workspace by email
    */
-  private async findUserByEmail(email: string): Promise<string | null> {
+  async findUserByEmail(email: string): Promise<string | null> {
     try {
       const response = await this.client.users.lookupByEmail({ email });
       return response.user?.id || null;