Add RevenueCat Capacitor IAP; keep Stripe for web billing.

Native builds purchase/restore via RevenueCat; browser keeps Stripe
checkout/portal. Billing history shows store ledger rows from backend
webhooks. Closes #100.

llm-fe/MONETIZATION.md

@@ -0,0 +1,40 @@
+# Monetization (web Stripe + native RevenueCat)
+
+## Channels
+
+| Runtime | Checkout / manage | Ledger |
+|---------|-------------------|--------|
+| Web (`!isNativePlatform`) | Stripe Checkout + Customer Portal via `/finance/...` | Stripe webhooks |
+| Native Capacitor | RevenueCat IAP (`@revenuecat/purchases-capacitor`) | RevenueCat webhooks → same invoice/payment tables |
+
+FE displays whatever `/finance/invoices/` (and subscription) returns after refresh. Provider badge uses `provider` + `revenuecat_store`.
+
+## App user id
+
+RevenueCat `appUserID` = Django user pk as string.
+
+1. Prefer `id` from `/user/get/` (serializer returns all model fields).
+2. Else JWT `user_id` claim (`SIMPLE_JWT.USER_ID_CLAIM`).
+
+Backend webhook resolver accepts numeric pk (or email fallback).
+
+## Mobile env keys
+
+In `.env.mobile`:
+
+- `REACT_APP_REVENUECAT_APPLE_API_KEY`
+- `REACT_APP_REVENUECAT_GOOGLE_API_KEY`
+- `REACT_APP_REVENUECAT_OFFERING_ID` (optional)
+
+Package identifiers in the RC dashboard should include plan slugs (e.g. `founders`) so `purchasePlan(planSlug)` can match packages/products. Optional backend `REVENUECAT_PRODUCT_PLAN_MAP` maps product id → plan slug.
+
+## Auth hooks
+
+- `Purchases.configure` once on native (first purchase / logIn).
+- `logIn(appUserID)` after SignIn / SignUp / AuthCallback / AccountContext load.
+- `logOut` on Header2 sign-out and Delete account.
+
+## Related
+
+- Issue #100 (FE) + companion backend RevenueCat webhook PR
+- [`ANDROID.md`](ANDROID.md) / [`IOS.md`](IOS.md) for store builds