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/package.json

@@ -30,6 +30,7 @@
     "expo-status-bar": "~2.0.0",
     "react": "18.3.1",
     "react-native": "0.76.9",
+    "react-native-purchases": "^8.2.0",
     "react-native-safe-area-context": "4.12.0",
     "react-native-screens": "~4.4.0",
     "react-native-url-polyfill": "^2.0.0"