feat(mobile): integrate react-native-purchases (RevenueCat)

- Purchases.configure() runs with appUserID = restaurantId at every
  point the id becomes known (login resolve, cold-start rehydration,
  onboarding restaurant creation) — this must match what the backend
  webhook expects (see apps/api/src/routes/subscription.ts).
- New account/subscription.tsx paywall: current entitlement status,
  offering packages with real store prices, purchase, restore.
- Linked from the account screen.

Native module — requires an EAS/dev-client build, not plain Expo Go.

apps/mobile/src/lib/purchases.ts

@@ -0,0 +1,37 @@
+import { Platform } from "react-native";
+import Purchases, { type CustomerInfo, type PurchasesOffering } from "react-native-purchases";
+
+const API_KEY = process.env.EXPO_PUBLIC_REVENUECAT_API_KEY;
+export const PRO_ENTITLEMENT_ID = "Menul.io Pro";
+
+let configuredForRestaurantId: string | null = null;
+
+// appUserID is the restaurant id — the backend webhook (POST
+// /subscription/webhook) maps RevenueCat's app_user_id straight to
+// subscriptions.restaurant_id, so these must always match.
+export function configurePurchases(restaurantId: string) {
+  if (!API_KEY) {
+    console.warn("EXPO_PUBLIC_REVENUECAT_API_KEY missing — purchases disabled");
+    return;
+  }
+  if (configuredForRestaurantId === restaurantId) return;
+
+  Purchases.configure({ apiKey: API_KEY, appUserID: restaurantId });
+  if (Platform.OS !== "web") {
+    Purchases.setLogLevel(__DEV__ ? Purchases.LOG_LEVEL.DEBUG : Purchases.LOG_LEVEL.WARN);
+  }
+  configuredForRestaurantId = restaurantId;
+}
+
+export async function getCurrentOffering(): Promise<PurchasesOffering | null> {
+  const offerings = await Purchases.getOfferings();
+  return offerings.current;
+}
+
+export async function getCustomerInfo(): Promise<CustomerInfo> {
+  return Purchases.getCustomerInfo();
+}
+
+export function isProActive(info: CustomerInfo): boolean {
+  return info.entitlements.active[PRO_ENTITLEMENT_ID] !== undefined;
+}