fix(db): RESTRICT delete of a Payment that has a FinancialBreakdown (H4)

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>

src/Settlement/Entity/FinancialBreakdown.php

@@ -26,7 +26,7 @@ class FinancialBreakdown
     private string $uuid;
 
     #[ORM\ManyToOne(targetEntity: Payment::class)]
-    #[ORM\JoinColumn(name: 'payment_id', referencedColumnName: 'id', nullable: false, onDelete: 'CASCADE')]
+    #[ORM\JoinColumn(name: 'payment_id', referencedColumnName: 'id', nullable: false, onDelete: 'RESTRICT')]
     private Payment $payment;
 
     #[ORM\Column(type: 'string', length: 20)]