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/lib/render-service.ts

@@ -0,0 +1,62 @@
+/**
+ * Render-engine kill switch. Admins can disable new renders (e.g. when no render
+ * node is available) via the `render_service` Website Setting; the studio then
+ * blocks "start render" and shows a localized "unavailable until <date>" message.
+ *
+ * Stored as one jsonb setting — no backend/migration. Reads fail OPEN (renders
+ * stay enabled) so a settings hiccup never blocks a working render farm.
+ */
+import { gatewayUrl } from "@/lib/api/gateway";
+
+export const RENDER_SERVICE_KEY = "render_service";
+
+export interface RenderServiceStatus {
+  enabled: boolean;
+  messageFa?: string;
+  messageEn?: string;
+  untilDate?: string; // ISO date (optional)
+}
+
+export async function fetchRenderServiceStatus(): Promise<RenderServiceStatus> {
+  try {
+    const res = await fetch(gatewayUrl("/v1/settings/"), {
+      cache: "no-store",
+      headers: { Accept: "application/json" },
+    });
+    if (!res.ok) return { enabled: true };
+    const rows = (await res.json()) as Array<{ key: string; value: string }>;
+    const row = Array.isArray(rows) ? rows.find((r) => r.key === RENDER_SERVICE_KEY) : null;
+    if (!row?.value) return { enabled: true };
+    const v = typeof row.value === "string" ? JSON.parse(row.value) : row.value;
+    return {
+      enabled: v?.enabled !== false,
+      messageFa: v?.messageFa ?? undefined,
+      messageEn: v?.messageEn ?? undefined,
+      untilDate: v?.untilDate ?? undefined,
+    };
+  } catch {
+    return { enabled: true }; // fail open
+  }
+}
+
+/** Pick the localized message, appending the "until" date when present. */
+export function renderServiceMessage(
+  status: RenderServiceStatus,
+  locale: string,
+  fallback: string,
+): string {
+  const base = (locale === "fa" ? status.messageFa : status.messageEn) || status.messageEn || status.messageFa || fallback;
+  if (status.untilDate) {
+    try {
+      const d = new Intl.DateTimeFormat(locale === "fa" ? "fa-IR" : "en-US", {
+        year: "numeric",
+        month: "long",
+        day: "numeric",
+      }).format(new Date(status.untilDate));
+      return `${base} (${d})`;
+    } catch {
+      return base;
+    }
+  }
+  return base;
+}