Grant mobile plans from verified App Store and Play purchases (#78) (#79)

## Summary
- Mobile access is granted by `POST /api/finance/store/confirm/` after the app finishes a StoreKit or Play Billing purchase.
- App Store signed transactions are checked against Apple Root CA G3. Play purchase tokens are checked with the Play Developer API.
- The RevenueCat webhook no longer grants a plan. Stripe Checkout and the Stripe webhook stay for the website.
- A purchase account token must match the signed-in user, and a transaction already linked to another account is rejected. Access ends when the verified period end passes unless the app confirms again.

Client: ai_ml_operations/chat_web_app#122

## Config
- `APPLE_IAP_BUNDLE_ID` (default `ai.hesychia.chat`)
- `GOOGLE_PLAY_PACKAGE_NAME`
- `GOOGLE_PLAY_SERVICE_ACCOUNT_JSON` (JSON or a file path)
- Plan `revenuecat_product_id` or `REVENUECAT_PRODUCT_PLAN_MAP` maps the store product to a plan

## Test plan
- [x] `python manage.py test monetization.tests.test_store_purchases monetization.tests.test_revenuecat monetization.tests.test_plans_quotas`
- [ ] Confirm an App Store sandbox transaction and a Play license-test purchase against a configured backend
- [ ] Confirm a token already linked to another user is rejected
- [ ] Confirm `GET /subscription/` returns `store_account_token` and flips a lapsed store sub to canceled

Reviewed-on: #79

llm_be/monetization/models.py

@@ -134,6 +134,8 @@ class UserSubscription(TimeInfoBase):
         NONE = "none", "None"
         STRIPE = "stripe", "Stripe"
         REVENUECAT = "revenuecat", "RevenueCat"
+        APP_STORE = "app_store", "App Store"
+        PLAY_STORE = "play_store", "Play Store"
         BACKER = "backer", "Backer"
         ADMIN = "admin", "Admin"
 
@@ -214,6 +216,8 @@ class Invoice(TimeInfoBase):
     class Provider(models.TextChoices):
         STRIPE = "stripe", "Stripe"
         REVENUECAT = "revenuecat", "RevenueCat"
+        APP_STORE = "app_store", "App Store"
+        PLAY_STORE = "play_store", "Play Store"
 
     class Status(models.TextChoices):
         DRAFT = "draft", "Draft"
@@ -308,6 +312,8 @@ class Payment(TimeInfoBase):
     class Provider(models.TextChoices):
         STRIPE = "stripe", "Stripe"
         REVENUECAT = "revenuecat", "RevenueCat"
+        APP_STORE = "app_store", "App Store"
+        PLAY_STORE = "play_store", "Play Store"
 
     class Status(models.TextChoices):
         PENDING = "pending", "Pending"