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/plugins/check-slack-joins.js

@@ -1,29 +0,0 @@
-// server/plugins/check-slack-joins.js
-import { checkSlackJoins } from '../utils/checkSlackJoins.js'
-
-const INTERVAL_MS = 3600000 // 1 hour
-
-export default defineNitroPlugin(() => {
-  // Don't run in test environment
-  if (process.env.NODE_ENV === 'test') return
-
-  const config = useRuntimeConfig()
-  const token = config.slackBotToken
-
-  if (!token) {
-    console.warn('[check-slack-joins] No Slack bot token configured, skipping background job')
-    return
-  }
-
-  async function run() {
-    try {
-      await checkSlackJoins(token)
-    } catch (err) {
-      console.error('[check-slack-joins] Unhandled error:', err.message || err)
-    }
-  }
-
-  // Run immediately on server start, then every hour
-  run()
-  setInterval(run, INTERVAL_MS)
-})