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/utils/slack-cleanup.test.js

@@ -0,0 +1,116 @@
+// Spec: docs/specs/wave-based-slack-onboarding.md
+// Test plan: docs/specs/wave-based-slack-onboarding-tests.md §8
+//
+// Static-source / filesystem assertions for the dead-code removal &
+// archival checklist. These don't require booting any handler — they
+// grep the working tree.
+//
+// SCAFFOLD: the whole suite is `.skip`ed until removal is done. As each
+// chunk lands, unskip the matching `it`.
+
+import { describe, it, expect } from 'vitest'
+import { readFileSync, existsSync, readdirSync } from 'node:fs'
+import { resolve } from 'node:path'
+
+const repoRoot = resolve(import.meta.dirname, '../../..')
+
+function read(rel) {
+  const path = resolve(repoRoot, rel)
+  return existsSync(path) ? readFileSync(path, 'utf-8') : null
+}
+
+function walk(dir, out = []) {
+  for (const entry of readdirSync(resolve(repoRoot, dir), { withFileTypes: true })) {
+    const rel = `${dir}/${entry.name}`
+    if (entry.isDirectory()) {
+      if (entry.name === 'node_modules' || entry.name.startsWith('.')) continue
+      if (entry.name === '_archive') continue
+      walk(rel, out)
+    } else if (/\.(js|ts|vue|mjs)$/.test(entry.name)) {
+      out.push(rel)
+    }
+  }
+  return out
+}
+
+describe('Slack onboarding — dead code removal (§8)', () => {
+  it('no admin.users.invite calls anywhere (8.1)', () => {
+    const files = walk('server').concat(walk('app'))
+    const hits = files.filter((f) => {
+      const src = read(f)
+      return src && /admin\.users\.invite/.test(src)
+    })
+    expect(hits).toEqual([])
+  })
+
+  it('inviteUserToSlack removed from slack.ts (8.2)', () => {
+    const src = read('server/utils/slack.ts')
+    expect(src).not.toMatch(/\binviteUserToSlack\b/)
+  })
+
+  it('findUserByEmail is callable (public or via wrapper) (8.3)', () => {
+    const src = read('server/utils/slack.ts') ?? ''
+    // Either the keyword `private` is removed from its declaration,
+    // OR a public wrapper exists.
+    const declLine = src.split('\n').find((l) => /findUserByEmail/.test(l)) ?? ''
+    const isPrivate = /\bprivate\b/.test(declLine)
+    const hasPublicWrapper = /export\s+(async\s+)?function\s+findUserByEmail/.test(src)
+      || /findUserByEmail\s*[:=]\s*async/.test(src)
+    expect(isPrivate && !hasPublicWrapper).toBe(false)
+  })
+
+  it('notifyNewMember retained (8.4)', () => {
+    const src = read('server/utils/slack.ts') ?? ''
+    expect(src).toMatch(/\bnotifyNewMember\b/)
+  })
+
+  it('checkSlackJoins archived, not at original path (8.5)', () => {
+    expect(existsSync(resolve(repoRoot, 'server/utils/checkSlackJoins.js'))).toBe(false)
+    // Any unloaded path is acceptable; check a couple of plausible ones.
+    const archived =
+      existsSync(resolve(repoRoot, 'server/utils/_archive/checkSlackJoins.js')) ||
+      existsSync(resolve(repoRoot, 'server/_archive/utils/checkSlackJoins.js'))
+    expect(archived).toBe(true)
+  })
+
+  it('no active cron/Nitro registration triggers checkSlackJoins (8.6)', () => {
+    const candidates = [
+      'nitro.config.ts',
+      'nitro.config.js',
+      'nuxt.config.ts',
+      'nuxt.config.js'
+    ].map(read).filter(Boolean)
+    const tasksDir = resolve(repoRoot, 'server/tasks')
+    const taskFiles = existsSync(tasksDir) ? walk('server/tasks') : []
+    const allSrc = candidates.concat(taskFiles.map(read).filter(Boolean)).join('\n')
+    expect(allSrc).not.toMatch(/checkSlackJoins/)
+  })
+
+  it('adminAlerts no longer queries the legacy invite-status field (8.7)', () => {
+    const src = read('server/utils/adminAlerts.js') ?? ''
+    expect(src).not.toMatch(new RegExp('slackInvite' + 'Status'))
+  })
+
+  it('adminAlerts.test.js drops the removed branch (8.8)', () => {
+    const src = read('tests/server/utils/adminAlerts.test.js') ?? ''
+    expect(src).not.toMatch(new RegExp('slackInvite' + 'Status'))
+    expect(src).not.toMatch(/detectSlackInviteFailed/)
+  })
+
+  it('check-slack-joins.test.js archived alongside source (8.9)', () => {
+    expect(existsSync(resolve(repoRoot, 'tests/server/tasks/check-slack-joins.test.js'))).toBe(false)
+  })
+
+  it('zero references to the legacy invite-status field repo-wide (8.10)', () => {
+    const files = walk('server').concat(walk('app')).concat(walk('tests'))
+    const hits = files.filter((f) => {
+      const src = read(f)
+      return src && new RegExp('slackInvite' + 'Status').test(src)
+    })
+    expect(hits).toEqual([])
+  })
+
+  it.todo('migration / cleanup story for dev/staging documented (8.11)')
+
+  it.todo('notifyNewMember invitationStatus arg uses canonical value at all call sites (8.12)')
+})