Add admin Stripe invoices and recurring customer subscriptions.

Closes #23: one-off hosted invoices, monthly Checkout subscriptions, pay-link email, and webhook status sync for admins.

.env.example

@@ -33,6 +33,13 @@ EMAIL_HOST_USER=
 EMAIL_HOST_PASSWORD=
 EMAIL_PORT=2525
 EMAIL_USE_TLS=true
+# DEFAULT_FROM_EMAIL=AI ML Operations, LLC <info@aimloperations.com>
+
+# Stripe (test keys from https://dashboard.stripe.com/test/apikeys)
+STRIPE_SECRET_KEY=
+STRIPE_PUBLISHABLE_KEY=
+# From `stripe listen` or Dashboard → Developers → Webhooks
+STRIPE_WEBHOOK_SECRET=
 
 # Gunicorn
 GUNICORN_WORKERS=2