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/signals.py

@@ -0,0 +1,8 @@
+"""Finance app signals."""
+
+
+def seed_plans_on_migrate(sender, **kwargs):
+    """Ensure the subscription catalog exists after migrate."""
+    from finance.services.plans import seed_subscription_plans
+
+    seed_subscription_plans(update_existing=False)