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>

tests/Settlement/FinancialBreakdownIntegrityTest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace App\Tests\Settlement;
+
+use App\Payment\Entity\Payment;
+use App\Settlement\Entity\FinancialBreakdown;
+use App\Tests\ApiTestCase;
+use Doctrine\DBAL\Exception\ForeignKeyConstraintViolationException;
+
+/**
+ * A FinancialBreakdown is an immutable ledger row. Deleting the Payment it
+ * settles must be blocked (ON DELETE RESTRICT) rather than silently cascading
+ * the ledger row away.
+ */
+class FinancialBreakdownIntegrityTest extends ApiTestCase
+{
+    public function testDeletingPaymentWithBreakdownIsRestricted(): void
+    {
+        $user    = $this->createUser();
+        $payment = new Payment($user, 100_000, 'mellat', Payment::TYPE_APPOINTMENT);
+        $this->em->persist($payment);
+
+        $breakdown = new FinancialBreakdown(
+            $payment,
+            FinancialBreakdown::SOURCE_APPOINTMENT,
+            $user,
+            100_000, 0, '0.00', 0, 100_000, '10.00', 10_000, 90_000,
+            null, null, null,
+        );
+        $this->em->persist($breakdown);
+        $this->em->flush();
+
+        $this->em->remove($payment);
+        $this->expectException(ForeignKeyConstraintViolationException::class);
+        $this->em->flush();
+    }
+}