feat: Setup projet Classeo avec infrastructure Docker et architecture DDD

Configure l'environnement de développement complet avec Docker Compose,
structure DDD 4 Bounded Contexts, et pipeline CI/CD GitHub Actions.

Corrections compatibilité CI:
- Symfony 8 nécessite monolog-bundle ^4.0 (la v3.x ne supporte que jusqu'à Symfony 7)
- ESLint v9 nécessite flat config (eslint.config.js) - le format .eslintrc.cjs est obsolète

backend/tests/Unit/Shared/Domain/EntityIdTest.php

@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Unit\Shared\Domain;
+
+use App\Shared\Domain\EntityId;
+use PHPUnit\Framework\TestCase;
+use Ramsey\Uuid\Uuid;
+
+final class EntityIdTest extends TestCase
+{
+    public function testGenerateCreatesValidUuid(): void
+    {
+        $id = TestEntityId::generate();
+
+        $this->assertInstanceOf(TestEntityId::class, $id);
+        $this->assertTrue(Uuid::isValid((string) $id));
+    }
+
+    public function testFromStringCreatesEntityIdFromValidUuid(): void
+    {
+        $uuid = '550e8400-e29b-41d4-a716-446655440000';
+        $id = TestEntityId::fromString($uuid);
+
+        $this->assertSame($uuid, (string) $id);
+    }
+
+    public function testEqualsReturnsTrueForSameValue(): void
+    {
+        $uuid = '550e8400-e29b-41d4-a716-446655440000';
+        $id1 = TestEntityId::fromString($uuid);
+        $id2 = TestEntityId::fromString($uuid);
+
+        $this->assertTrue($id1->equals($id2));
+    }
+
+    public function testEqualsReturnsFalseForDifferentValue(): void
+    {
+        $id1 = TestEntityId::generate();
+        $id2 = TestEntityId::generate();
+
+        $this->assertFalse($id1->equals($id2));
+    }
+
+    public function testToStringReturnsUuidString(): void
+    {
+        $uuid = '550e8400-e29b-41d4-a716-446655440000';
+        $id = TestEntityId::fromString($uuid);
+
+        $this->assertSame($uuid, (string) $id);
+    }
+}
+
+// Test concrete implementation
+final readonly class TestEntityId extends EntityId
+{
+}