RevenueCat Capacitor IAP; keep Stripe on web (#100) (#102)

## Summary
- Native Capacitor: RevenueCat purchase/restore (`@revenuecat/purchases-capacitor`)
- Web: Stripe checkout/portal unchanged
- Billing history shows store rows (`provider` / `revenuecat_store`)
- RC `logIn`/`logOut` on auth lifecycle; appUserID = user pk

Closes #100. Depends on backend [chat_backend#68](ai_ml_operations/chat_backend#68) / monetization PR.

## Test plan
- [x] Unit tests: BillingSection, revenueCat, finance helpers, SignIn/Up, AuthCallback, Header2, DeleteAccount (38 OK)
- [ ] Fill `REACT_APP_REVENUECAT_*` keys in `.env.mobile`
- [ ] Align RC offerings/products with plan slugs
- [ ] `npm run build:mobile` + `cap sync`; sandbox purchase on Android/iOS
- [ ] Confirm web Stripe checkout/portal still work
- [ ] After RC webhook, Account → Billing history shows store invoiceReviewed-on: #102

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 `/monetization/...` | Stripe webhooks |
+| Native Capacitor | RevenueCat IAP (`@revenuecat/purchases-capacitor`) | RevenueCat webhooks → same invoice/payment tables |
+
+FE displays whatever `/monetization/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