fix(subscription-client): add missing X-Product-Id header to API requests

Aligns with referral-client, org-client, and marketplace-client which
all send this header. The billing API likely requires it for product
scoping.

Made-with: Cursor

packages/subscription-client/src/index.ts

@@ -50,6 +50,7 @@ export function createSubscriptionClient(
     const token = opts.getAccessToken();
     const headers = new Headers(init?.headers);
     headers.set("Authorization", `Bearer ${token}`);
+    headers.set("X-Product-Id", opts.productId);
     if (!headers.has("Content-Type") && init?.body !== undefined) {
       headers.set("Content-Type", "application/json");
     }