chore(slack): remove dead invite path, archive checkSlackJoins poller

Wave-based onboarding makes the auto-invite + polling path obsolete.

- Removes SlackService.inviteUserToSlack — admins now send invites
  through Slack's UI and flip the flag in our admin endpoint.
- Removes the slack_invite_failed admin alert + its detector. The
  alert no longer has a meaningful trigger (we don't attempt invites).
- Archives server/utils/checkSlackJoins.js (and its test) under
  _archive/ in case the polling pattern is needed again post-pilot.
- Deletes the Nitro plugin that scheduled checkSlackJoins on boot
  + hourly. Nothing in nitro.config / nuxt.config / package.json
  registered it elsewhere.
- Drops the slack_invite_failed branch from adminAlerts.test; the
  enum slug stays in adminAlertDismissal so historical dismissal
  rows continue to validate.

notifyNewMember (vetting-channel notification) and findUserByEmail
(used by the auto-flag helper) are retained.

server/utils/adminAlerts.js

@@ -17,7 +17,6 @@ export const ALERT_THRESHOLDS = {
 // Single source of truth for alert presentation. Used by detectors AND the
 // dismissed-list endpoint (which has no access to a detector run's output).
 export const ALERT_METADATA = {
-  slack_invite_failed: { title: 'Slack invites failed', severity: 'critical' },
   no_slack_handle_week: { title: 'Active members without a Slack handle', severity: 'attention' },
   stuck_pending_payment: { title: 'Members stuck in pending payment', severity: 'attention' },
   member_suspended: { title: 'Suspended members', severity: 'attention' },
@@ -62,18 +61,6 @@ function memberItem(member, sublabel) {
   }
 }
 
-export async function detectSlackInviteFailed() {
-  await connectDB()
-  const members = await Member
-    .find({ slackInviteStatus: 'failed' })
-    .select('name email')
-    .lean()
-  return {
-    ...alertShell('slack_invite_failed'),
-    items: members.map((m) => memberItem(m))
-  }
-}
-
 export async function detectNoSlackHandleAfterWeek() {
   await connectDB()
   const cutoff = daysAgo(ALERT_THRESHOLDS.NO_SLACK_DAYS)
@@ -257,7 +244,6 @@ export async function detectPendingTagSuggestions() {
 }
 
 const DETECTORS = [
-  detectSlackInviteFailed,
   detectNoSlackHandleAfterWeek,
   detectStuckPendingPayment,
   detectSuspendedMembers,