Stripe Webhook

Handle Stripe webhook events.

Subscription + auto-refill model:

• checkout.session.completed (mode=subscription): save customer/sub IDs + PM
• customer.subscription.updated: handle tier upgrade/downgrade
• customer.subscription.deleted: cancel — clear active plan
• invoice.paid: grant monthly block credits + reset usage counters
• invoice.payment_failed: mark plan inactive, notify user
• payment_intent.succeeded (auto_refill): informational log only (credits are granted synchronously by the auto_refill service)

Expert payouts:

• account.updated, payout.paid, payout.failed (unchanged)