Add task assignment notification system

- Introduced `AssignTaskEvent`, `TaskAssignEventSubscriber`, and `AssignTaskNotificationFlagProvider` for task assignment notifications
- WIP : Added notification templates for task assignment title and content
- Updated `SingleTaskController` to dispatch `AssignTaskEvent`
- Adjusted translations and service configurations accordingly

src/Bundle/ChillTaskBundle/Event/AssignTaskEvent.php

@@ -0,0 +1,35 @@
+<?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\TaskBundle\Event;
+
+use Chill\MainBundle\Entity\User;
+use Chill\TaskBundle\Entity\SingleTask;
+use Symfony\Contracts\EventDispatcher\Event;
+
+class AssignTaskEvent extends Event
+{
+    final public const PERSIST = 'chill_task.assign_task';
+    public function __construct(
+        private readonly SingleTask $task,
+        private readonly User $assignedUser,
+    ) {}
+
+    public function getTask(): SingleTask
+    {
+        return $this->task;
+    }
+
+    public function getAssignedUser(): User
+    {
+        return $this->assignedUser;
+    }
+}