DC-048: multi-user bootstrap + admin invites (opt-in)

Implements the user-store + invite-store + admin routes. The whole
system is opt-in via siteConfig.authProviders.email.enabled = true;
single-user TOTP-only installs see zero behavior change.

Backend:
- src/security/user-store.js: users + allowlist + bootstrap sentinel,
  atomic writes, last-admin protection, defensive dataDir resolver.
- src/security/invite-store.js: single-use tokens (SHA-256 hashed on
  disk), TTL, auto-prune, defensive dataDir resolver.
- routes/auth/admin.js: /me, /admin/users (CRUD), /admin/allowlist,
  /admin/invites (CRUD), public /invites/:token (peek + accept).
- routes/auth/index.js: wires userStore, gates admin router on
  email auth being enabled.
- src/auth/providers/email.js: verify() enforces allowlist, creates
  user record, tags req.user; default-enabled flipped to opt-in.
- src/auth/providers/totp.js: bootstraps system@totp.local admin on
  first verify so current DNS2 operator shows in /admin/users.
- src/security/audit-logger.js: middleware adds userId/userEmail/
  userRole/viaProvider to log details when req.user is tagged.
- PUBLIC_ROUTES + CSRF allowlists updated for invite redemption.

Frontend:
- status/js/admin.js: modal overlay with users list (role-edit,
  delete), invite form (email/role/TTL), copy-link button,
  outstanding-invites list with revoke. Exports window.AdminPanel.
- status/js/core/init.js: calls AdminPanel.attachTrigger so the
  Admin button only appears when /me returns isAdmin=true.

Tests: 35 new tests across 3 files (user-store, invite-store, auth
multistore integration). Full suite: 1298/1298 passing.

Docs: BACKLOG.md marks DC-048 done. CHANGELOG.md [Unreleased]
section gets the DC-048 entry.

dashcaddy-api/__tests__/public-routes-drift.test.js

@@ -288,9 +288,20 @@ describe('Public-routes allowlist drift (prevents DC-012-style dead entries)', (
 
   describe('No stale PUBLIC_ROUTES entries (the DC-012 failure mode)', () => {
     test('every PUBLIC_ROUTES entry matches an actual mounted route', () => {
+      // DC-048: invite routes are only mounted when the operator has
+      // enabled email auth (siteConfig.authProviders.email.enabled === true).
+      // The aggregator factory gates this on a non-proxied config flag, so
+      // the router walker in this test (which runs with stub deps) doesn't
+      // see them mounted. They're not stale — they're conditional. Same
+      // for any future provider-conditional mount.
+      const conditionalMounts = new Set([
+        '/api/v1/auth/invites/:token',
+        '/api/v1/auth/invites/:token/accept',
+      ]);
       const stale = [];
       for (const entry of publicRoutes) {
         if (entry.endsWith('/')) continue; // prefix matches, skip
+        if (conditionalMounts.has(entry)) continue; // gated by config flag
         if (!mountedRoutes.has(entry)) stale.push(entry);
       }
       expect(stale).toEqual([]);