Processed 5 Sentry errors from payments-api (past 24h):

1. TypeError: Cannot read property 'last4' of null (1,892 events)
   Root cause: Stripe webhook delivers `payment_method: null` for
   bank transfer payments. CheckoutReceipt.tsx:34 destructures
   without a null check.
   PR #612: Add null safety to CheckoutReceipt, show "Bank Transfer"
   fallback for non-card payments.

2. TimeoutError: Query timeout after 30s on /api/invoices (743 events)
   Root cause: N+1 query in InvoiceService.getMonthly() — each
   line item triggers a separate product lookup.
   PR #613: Add eager loading for invoice line items with
   Sequelize `include`.

3. RangeError: Maximum call stack size exceeded (412 events)
   Root cause: Circular reference in refund.toJSON() when a
   refund references its parent transaction which references
   the refund.
   PR #614: Break circular ref with a custom serializer,
   add max-depth test.

4-5. Two lower-frequency validation errors — PRs #615, #616.