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

The app confirms a StoreKit signed transaction or Play purchase token. The backend checks it with Apple or Google before writing the ledger, and the RevenueCat webhook no longer grants access.

llm_be/monetization/services/plans.py

@@ -247,6 +247,8 @@ def needs_checkout(user) -> bool:
         UserSubscription.Source.ADMIN,
         UserSubscription.Source.STRIPE,
         UserSubscription.Source.REVENUECAT,
+        UserSubscription.Source.APP_STORE,
+        UserSubscription.Source.PLAY_STORE,
     ):
         return False
     return True
@@ -457,8 +459,9 @@ def assign_plan_from_revenuecat(
     cancel_at_period_end: bool | None = None,
     current_period_end=None,
     keep_existing_plan_if_unknown: bool = False,
+    source: str | None = None,
 ) -> UserSubscription:
-    """Assign a catalog plan from a RevenueCat store purchase event."""
+    """Assign a catalog plan from a verified store purchase."""
     seed_subscription_plans(update_existing=False)
     existing = (
         UserSubscription.objects.filter(user=user).select_related("plan").first()
@@ -493,7 +496,7 @@ def assign_plan_from_revenuecat(
     sub = assign_plan(
         user,
         plan=plan,
-        source=UserSubscription.Source.REVENUECAT,
+        source=source or UserSubscription.Source.REVENUECAT,
         status=status,
         revenuecat_original_transaction_id=revenuecat_original_transaction_id or "",
         log_auth_event=False,