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/app/(onboarding)/restaurant.tsx

@@ -4,6 +4,7 @@ import { ActivityIndicator, Pressable, ScrollView, Text, TextInput, View } from
 import { Ionicons } from "@expo/vector-icons";
 import { api } from "@/lib/api";
 import { setActiveRestaurant } from "@/lib/active-restaurant";
+import { configurePurchases } from "@/lib/purchases";
 
 interface CreateRestaurantResponse {
   restaurant: { id: string; slug: string };
@@ -34,6 +35,7 @@ export default function OnboardingRestaurantStep() {
         menuId: menu.id,
         slug: restaurant.slug,
       });
+      configurePurchases(restaurant.id);
 
       router.replace("/(onboarding)/scan");
     } catch (err) {