feat: /spending page — AI cost dashboard (aiprimetech + routerai)

- app/spending/page.js: расходы по периодам, разбивка по провайдерам и типам
- app/api/usage/summary/route.js: прокси к engine /api/usage/summary
- Header.js: ссылка «Расходы» для admin (TrendingUp иконка)

components/Header.js

@@ -1,7 +1,7 @@
 'use client';
 import Link from 'next/link';
 import { useRouter } from 'next/navigation';
-import { Sparkles, LogOut, Settings2, CalendarDays } from 'lucide-react';
+import { Sparkles, LogOut, Settings2, CalendarDays, TrendingUp } from 'lucide-react';
 import ThemeToggle from './ThemeToggle';
 
 export default function Header({ user }) {
@@ -22,6 +22,12 @@ export default function Header({ user }) {
             <CalendarDays className="w-4 h-4" />
             <span>Календарь</span>
           </Link>
+          {user?.isAdmin && (
+            <Link href="/spending" className="btn-ghost px-3 py-1.5 text-sm flex items-center gap-1.5">
+              <TrendingUp className="w-4 h-4" />
+              <span>Расходы</span>
+            </Link>
+          )}
         </nav>
         <div className="flex items-center gap-2">
           {user?.isAdmin && (