Add multi-plan subscriptions, quotas, and token usage APIs

Implements #16/#17/#36: Founders/Standard/Pro/Business/Backer catalog,
Backer email whitelist, prompt-window + token-period gates, and
tokens_in/out on conversation/prompt + subscription usage APIs.

llm_be/finance/urls.py

@@ -5,7 +5,9 @@ from finance.views import (
     CreateCheckoutSessionView,
     InvoiceListView,
     PaymentListView,
+    PlanListView,
     StripeWebhookView,
+    SubscriptionMeView,
 )
 
 urlpatterns = [
@@ -29,6 +31,16 @@ urlpatterns = [
         PaymentListView.as_view(),
         name="finance_payments",
     ),
+    path(
+        "plans/",
+        PlanListView.as_view(),
+        name="finance_plans",
+    ),
+    path(
+        "subscription/",
+        SubscriptionMeView.as_view(),
+        name="finance_subscription_me",
+    ),
     path(
         "webhooks/stripe/",
         StripeWebhookView.as_view(),