Add social login (Discord/Google) + batch-6 pages

Auth: NextAuth Discord + Google providers (enabled when env id+secret set);
OAuth signIn allowed only if a hotel account matches the email; jwt binds the
session to that account (id/rank/username). Login page gets social buttons.

Batch 6 (parallel agents): /friends (messenger_friendships), /guilds/[id]/forum
(threads), /admin/navigation (navigator config), /admin/maintenance (toggle
maintenance settings), /admin/alerts (alert_logs + send hotel alert via RCON).
Header (Friends) + admin nav (Alerts/Maintenance/Navigator) extended.

Verified: tsc exit 0, vitest 48/48, next build exit 0 (57 page routes).

src/app/admin/layout.tsx

@@ -32,6 +32,9 @@ export default async function AdminLayout({ children }: { children: ReactNode })
           <Link href="/admin/wordfilter">Word Filter</Link>
           <Link href="/admin/ip">IP Management</Link>
           <Link href="/admin/logs">Logs</Link>
+          <Link href="/admin/alerts">Alerts</Link>
+          <Link href="/admin/maintenance">Maintenance</Link>
+          <Link href="/admin/navigation">Navigator</Link>
           <Link href="/admin/teams">Teams</Link>
           <Link href="/admin/housekeeping">Housekeeping</Link>
           <Link href="/admin/permissions">Permissions</Link>