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/services/webhooks.py

@@ -11,6 +11,7 @@ from django.db import transaction
 from django.utils import timezone
 
 from finance.models import Invoice, Payment
+from finance.services.plans import assign_founders_from_stripe
 
 logger = logging.getLogger(__name__)
 User = get_user_model()
@@ -211,6 +212,11 @@ def handle_checkout_session_completed(session: dict[str, Any]) -> Invoice | None
             ),
             paid_at=timezone.now(),
         )
+    if session.get("payment_status") == "paid" or session.get("subscription"):
+        assign_founders_from_stripe(
+            user,
+            stripe_subscription_id=session.get("subscription") or "",
+        )
     return invoice
 
 
@@ -270,6 +276,10 @@ def handle_invoice_paid(stripe_invoice: dict[str, Any]) -> Invoice | None:
             stripe_charge_id=charge if isinstance(charge, str) else None,
             paid_at=paid_at,
         )
+    assign_founders_from_stripe(
+        user,
+        stripe_subscription_id=stripe_invoice.get("subscription") or "",
+    )
     return invoice