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.

.env.example

@@ -44,13 +44,15 @@ OAUTH_CALLBACK_BASE_URL=http://127.0.0.1:8001
 STRIPE_SECRET_KEY=
 STRIPE_PUBLISHABLE_KEY=
 STRIPE_WEBHOOK_SECRET=
-# Optional: pre-created Stripe Price ID. When empty, Checkout uses
-# SUBSCRIPTION_PRICE_* from settings.py ($10 USD / month by default).
+# Optional: pre-created Stripe Price ID for Founders. When empty, Checkout uses
+# SubscriptionPlan.price_cents / SUBSCRIPTION_PRICE_* ($10 USD / month Founders).
 STRIPE_PRICE_ID=
 # SUBSCRIPTION_PRICE_AMOUNT_CENTS=1000
 # SUBSCRIPTION_PRICE_CURRENCY=usd
 # SUBSCRIPTION_PRICE_INTERVAL=month
-# SUBSCRIPTION_PRODUCT_NAME=Chat Subscription
+# SUBSCRIPTION_PRODUCT_NAME=Founders
+# Enforce plan feature + prompt/token quotas on chat turns (default true).
+# ENFORCE_SUBSCRIPTION_GATES=true
 FRONTEND_BASE_URL=http://localhost:3000
 # STRIPE_CHECKOUT_SUCCESS_URL=http://localhost:3000/billing/success?session_id={CHECKOUT_SESSION_ID}
 # STRIPE_CHECKOUT_CANCEL_URL=http://localhost:3000/billing/cancel