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/components/site-header.tsx

@@ -97,6 +97,9 @@ export async function SiteHeader() {
                 {diamonds}
               </span>
             </div>
+            <Link className="nav-item" href="/friends">
+              Friends
+            </Link>
             <Link className="nav-item" href="/redeem">
               Redeem
             </Link>