Add unique constraint for definition_id and person_id in PersonIdentifier

- Update entity to include the new database-level unique constraint.
- Add migration script to apply the unique constraint and handle rollback.

src/Bundle/ChillPersonBundle/Entity/Identifier/PersonIdentifier.php

@@ -18,6 +18,7 @@ use Doctrine\ORM\Mapping as ORM;
 #[ORM\Entity]
 #[ORM\Table(name: 'chill_person_identifier')]
 #[ORM\UniqueConstraint(name: 'chill_person_identifier_unique', columns: ['definition_id', 'canonical'])]
+#[ORM\UniqueConstraint(name: 'chill_person_identifier_unique_person_definition', columns: ['definition_id', 'person_id'])]
 #[UniqueIdentifierConstraint]
 class PersonIdentifier
 {

src/Bundle/ChillPersonBundle/migrations/Version20250926124024.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace Chill\Migrations\Person;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20250926124024 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add unique constraint on person_identifier: only one identifier of each kind by person';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('CREATE UNIQUE INDEX chill_person_identifier_unique_person_definition ON chill_person_identifier (definition_id, person_id)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP INDEX chill_person_identifier_unique_person_definition');
+    }
+}