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.

tests/server/api/admin-members-slack-status.test.js

@@ -29,7 +29,6 @@ vi.mock('../../../server/utils/schemas.js', () => ({
 // Slack service must NOT be invoked from this endpoint.
 vi.mock('../../../server/utils/slack.ts', () => ({
   getSlackService: vi.fn().mockReturnValue({
-    inviteUserToSlack: vi.fn(),
     findUserByEmail: vi.fn()
   })
 }))
@@ -172,7 +171,6 @@ describe('PATCH /api/admin/members/[id]/slack-status', () => {
       await handler(makeEvent())
 
       const slack = getSlackService()
-      expect(slack.inviteUserToSlack).not.toHaveBeenCalled()
       expect(slack.findUserByEmail).not.toHaveBeenCalled()
     })
   })