fix(db): business-key unique constraints (M16-M19)

Add unique constraints (one migration, no dup data in either DB):
- users.email, users.national_code (M16) — NULLs still allowed.
- payments.gateway_token (M17).
- date_overrides (doctor_id, date) (M18) — was a non-unique index.
- financial_breakdowns (payment_id, source) (M19) — anti double-accounting.

Regression: tests/Database/UniqueConstraintsTest (4 duplicate-insert cases).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

migrations/Version20260628173151.php

@@ -0,0 +1,39 @@
+<?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 Version20260628173151 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 date_overrides DROP INDEX idx_date_overrides_doctor_date, ADD UNIQUE INDEX uniq_date_override_doctor_date (doctor_id, date)');
+        $this->addSql('CREATE UNIQUE INDEX uniq_breakdown_payment_source ON financial_breakdowns (payment_id, source)');
+        $this->addSql('CREATE UNIQUE INDEX UNIQ_65D29B3223882363 ON payments (gateway_token)');
+        $this->addSql('CREATE UNIQUE INDEX UNIQ_1483A5E9E7927C74 ON users (email)');
+        $this->addSql('CREATE UNIQUE INDEX UNIQ_1483A5E9D3C17DD2 ON users (national_code)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE date_overrides DROP INDEX uniq_date_override_doctor_date, ADD INDEX idx_date_overrides_doctor_date (doctor_id, date)');
+        $this->addSql('DROP INDEX uniq_breakdown_payment_source ON financial_breakdowns');
+        $this->addSql('DROP INDEX UNIQ_65D29B3223882363 ON payments');
+        $this->addSql('DROP INDEX UNIQ_1483A5E9E7927C74 ON users');
+        $this->addSql('DROP INDEX UNIQ_1483A5E9D3C17DD2 ON users');
+    }
+}