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)

app/api/usage/summary/route.js

@@ -0,0 +1,15 @@
+import { NextResponse } from 'next/server';
+import { requireUser } from '@/lib/session';
+import { engine } from '@/lib/engine';
+
+export async function GET(req) {
+  const user = await requireUser();
+  if (!user?.isAdmin) return NextResponse.json({ error: 'Forbidden' }, { status: 403 });
+  const { searchParams } = new URL(req.url);
+  try {
+    const data = await engine.usageSummary(Object.fromEntries(searchParams));
+    return NextResponse.json(data);
+  } catch (err) {
+    return NextResponse.json({ error: err.message }, { status: 500 });
+  }
+}