feat(admin): render-engine kill switch (block renders + show message)

Lets an admin disable rendering when no render node is available — users can't
start new renders and see a localized "service unavailable until <date>" message.

- Admin → فارم رندر → موتور رندر (RenderEngineAdmin): on/off toggle + fa/en message
  + optional Jalali "until" date; saved as one `render_service` Website Setting
  (jsonb) via /v1/settings — no backend change, no migration.
- lib/render-service.ts: fetchRenderServiceStatus (fail-open) + renderServiceMessage
  (locale + appends the date).
- Enforcement: POST /api/render returns 503 {code:render_disabled, messages} when off;
  studio render page reads GET /api/render/service on mount → disables "شروع رندر"
  and shows the banner, and handles the 503 on click.
- i18n: appAdminLayout.renderEngine (fa+en, parity 1045/1045). tsc + next build clean.
  Verified: disabled setting → /api/render/service returns enabled:false.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/[locale]/admin/layout.tsx

@@ -64,6 +64,7 @@ export default async function AdminLayout({
     {
       title: "فارم رندر",
       items: [
+        { href: "/admin/render-engine", label: t("renderEngine") },
         { href: "/admin/nodes", label: t("nodes") },
         { href: "/admin/node-fonts", label: t("nodeFonts") },
         { href: "/admin/renders", label: t("renderQueue") },