feat: inversion relation User→Projects, les projets pointent vers les utilisateurs

Le champ `users` est désormais sur le blueprint projet. Les blueprints
pochet/client perdent leur champ `projects`. La logique PHP (user-projects,
managers, controller, template, mark-all-read) lit project.users au lieu
de user.projects. Script de migration inclus.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/site/config/config.php

@@ -44,6 +44,7 @@ return [
     require(__DIR__ . '/routes/request-project-creation.php'),
     require(__DIR__ . '/routes/request-optimization-appointment.php'),
     require(__DIR__ . '/routes/migrate-notifications.php'),
+    require(__DIR__ . '/routes/migrate-user-projects.php'),
   ],
   'hooks' => [
     'page.create:after'  => require_once(__DIR__ . '/hooks/create-steps.php'),