The ledger FK used ON DELETE CASCADE on a non-nullable column, so deleting a
Payment silently destroyed its immutable financial breakdown rows. Switch to
RESTRICT — a settled payment can no longer be deleted out from under its ledger.
Regression: tests/Settlement/FinancialBreakdownIntegrityTest.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
