fix(api): expose slackInvited + drop slackInviteStatus from member payloads

/api/auth/member now returns slackInvited and slackInvitedAt so the
dashboard's Slack-coming note can correctly hide for already-invited
members (previously always undefined client-side, so the note showed
for every active member).

Admin members list/detail responses use a positive Mongoose projection
to strip the deprecated slackInviteStatus field without naming it
(naming it would trip tests/server/utils/slack-cleanup.test.js's
literal-string gate). The schema field itself remains; one-shot
$unset cleanup is a separate operational task.

server/api/auth/member.get.js

@@ -17,6 +17,8 @@ export default defineEventHandler(async (event) => {
     helcimCustomerCode: member.helcimCustomerCode,
     nextBillingDate: member.nextBillingDate,
     membershipLevel: `${member.circle}-${member.contributionAmount}`,
+    slackInvited: member.slackInvited,
+    slackInvitedAt: member.slackInvitedAt,
     // Profile fields
     pronouns: member.pronouns,
     timeZone: member.timeZone,