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/admin/members/[id].get.js

@@ -8,7 +8,8 @@ export default defineEventHandler(async (event) => {
 
   await connectDB()
 
-  const member = await Member.findById(memberId).lean()
+  const projection = Object.keys(Member.schema.paths).join(' ')
+  const member = await Member.findById(memberId).select(projection).lean()
   if (!member) {
     throw createError({ statusCode: 404, statusMessage: 'Member not found' })
   }