feat: Provisionner automatiquement un nouvel établissement

Lorsqu'un super-admin crée un établissement via l'interface, le système
doit automatiquement créer la base tenant, exécuter les migrations,
créer le premier utilisateur admin et envoyer l'invitation — le tout
de manière asynchrone pour ne pas bloquer la réponse HTTP.

Ce mécanisme rend chaque établissement opérationnel dès sa création
sans intervention manuelle sur l'infrastructure.

backend/composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "92b9472c96a59c314d96372c4094f185",
+    "content-hash": "a4b309e6d9d273339a5154a64a5f7361",
     "packages": [
         {
             "name": "api-platform/core",
@@ -4672,6 +4672,73 @@
             ],
             "time": "2024-09-25T14:21:43+00:00"
         },
+        {
+            "name": "symfony/expression-language",
+            "version": "v8.0.8",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/expression-language.git",
+                "reference": "b2a5fd3b7331ae10cc0ed75a28d64b25b67d2c7b"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/expression-language/zipball/b2a5fd3b7331ae10cc0ed75a28d64b25b67d2c7b",
+                "reference": "b2a5fd3b7331ae10cc0ed75a28d64b25b67d2c7b",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.4",
+                "symfony/cache": "^7.4|^8.0",
+                "symfony/service-contracts": "^2.5|^3"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Symfony\\Component\\ExpressionLanguage\\": ""
+                },
+                "exclude-from-classmap": [
+                    "/Tests/"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Fabien Potencier",
+                    "email": "fabien@symfony.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Provides an engine that can compile and evaluate expressions",
+            "homepage": "https://symfony.com",
+            "support": {
+                "source": "https://github.com/symfony/expression-language/tree/v8.0.8"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/nicolas-grekas",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2026-03-30T15:14:47+00:00"
+        },
         {
             "name": "symfony/filesystem",
             "version": "v8.0.1",