Use internal origin for runtime locale state

tests/middleware.test.ts

@@ -39,6 +39,8 @@ function createMockRequest(url: string) {
 describe("middleware locale runtime config", () => {
   beforeEach(() => {
     vi.resetModules();
+    process.env.NODE_ENV = "production";
+    delete process.env.SITE_RUNTIME_ORIGIN;
     createMiddlewareMock.mockReset();
     intlHandlerMock.mockReset();
     intlHandlerMock.mockReturnValue(NextResponse.next());
@@ -66,6 +68,10 @@ describe("middleware locale runtime config", () => {
 
     await middleware(request as never);
 
+    expect(fetch).toHaveBeenCalledWith(
+      new URL("http://127.0.0.1:3000/api/site/default-locale"),
+      expect.any(Object),
+    );
     expect(createMiddlewareMock).toHaveBeenCalledTimes(1);
     expect(createMiddlewareMock).toHaveBeenCalledWith(
       expect.objectContaining({
@@ -115,4 +121,29 @@ describe("middleware locale runtime config", () => {
     expect(response.headers.get("location")).toBe("https://example.com/coming-soon");
     expect(intlHandlerMock).not.toHaveBeenCalled();
   });
+
+  it("prefers the configured internal runtime origin when provided", async () => {
+    process.env.SITE_RUNTIME_ORIGIN = "http://app:3000";
+
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        ok: true,
+        json: async () => ({
+          defaultLocale: "en",
+          maintenanceEnabled: false,
+        }),
+      })),
+    );
+
+    const { default: middleware } = await import("../middleware");
+    const request = createMockRequest("https://example.com/");
+
+    await middleware(request as never);
+
+    expect(fetch).toHaveBeenCalledWith(
+      new URL("http://app:3000/api/site/default-locale"),
+      expect.any(Object),
+    );
+  });
 });