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>

migrations/Version20260628153855.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260628153855 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return '';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE financial_breakdowns DROP FOREIGN KEY `FK_D66D9FF44C3A3BB`');
+        $this->addSql('ALTER TABLE financial_breakdowns ADD CONSTRAINT FK_D66D9FF44C3A3BB FOREIGN KEY (payment_id) REFERENCES payments (id) ON DELETE RESTRICT');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE financial_breakdowns DROP FOREIGN KEY FK_D66D9FF44C3A3BB');
+        $this->addSql('ALTER TABLE financial_breakdowns ADD CONSTRAINT `FK_D66D9FF44C3A3BB` FOREIGN KEY (payment_id) REFERENCES payments (id) ON DELETE CASCADE');
+    }
+}