Add finance app with Stripe Checkout subscriptions (#21)

Introduce Invoice/Payment ledger models, $10/mo settings-backed pricing,
hosted Checkout + signed webhooks, Django admin, and authenticated list APIs.

.env.example

@@ -25,6 +25,21 @@ EMAIL_USE_TLS=true
 # Captcha (optional local)
 CAPTCHA_SECRET_KEY=
 
+# Stripe / finance (optional local — required for checkout + webhooks)
+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).
+STRIPE_PRICE_ID=
+# SUBSCRIPTION_PRICE_AMOUNT_CENTS=1000
+# SUBSCRIPTION_PRICE_CURRENCY=usd
+# SUBSCRIPTION_PRICE_INTERVAL=month
+# SUBSCRIPTION_PRODUCT_NAME=Chat Subscription
+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
+
 # Gunicorn / ASGI
 GUNICORN_WORKERS=2
 GUNICORN_BIND=0.0.0.0:8000