Stripe Webhook

Stripe webhook receiver. Verified via Stripe-Signature header — no user auth.

Handled events:

• checkout.session.completed → primary subscription provisioning
• customer.subscription.created → secondary provisioning (dashboard/manual subs)
• customer.subscription.updated → plan upgrades, downgrades, renewals
• customer.subscription.deleted → downgrade to free
• invoice.payment_failed → mark past_due (grace period, no immediate downgrade)
• invoice.payment_action_required → mark past_due
• setup_intent.succeeded → save payment method from card-setup flow
• payment_intent.succeeded → idempotent credit topup (async fallback)
• payment_intent.payment_failed → log payment failure