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/config/packages/messenger.yaml

@@ -0,0 +1,44 @@
+framework:
+    messenger:
+        # Uncomment this (and the failed transport below) to send failed messages to this transport for later handling.
+        failure_transport: failed
+
+        # Three buses: Command, Query, Event (CQRS + Event-driven)
+        default_bus: command.bus
+
+        buses:
+            command.bus:
+                default_middleware: true
+                middleware:
+                    - doctrine_transaction
+
+            query.bus:
+                default_middleware: true
+
+            event.bus:
+                default_middleware:
+                    allow_no_handlers: true
+
+        transports:
+            # https://symfony.com/doc/current/messenger.html#transport-configuration
+            async:
+                dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
+                options:
+                    exchange:
+                        name: classeo_messages
+                        type: topic
+                    queues:
+                        messages:
+                            binding_keys: ['#']
+                retry_strategy:
+                    max_retries: 3
+                    delay: 1000
+                    multiplier: 2
+                    max_delay: 60000
+
+            failed:
+                dsn: 'doctrine://default?queue_name=failed'
+
+        routing:
+            # Route your messages to the transports
+            # 'App\Message\YourMessage': async