[System] support realtime tasks distribution

[SagiEv]

Summary:
When a user has already loaded their task list, any tasks assigned afterward are not reflected in the UI because the task list remains cached. The user only sees the new tasks after a full session refresh (e.g., logout/login).
Add a real-time notification mechanism so that task assignment events trigger cache invalidation and a task list re-fetch on the client.

Issue: If a new task is assigned to a user after they have already loaded their task list, the frontend continues showing the cached task list and does not become aware of the newly assigned task. The user only sees the new task after a logout/login cycle (or another action that forces a refresh).

Expected Behavior: Users should be notified when their task list becomes stale due to new task assignments and the frontend should refresh the task list automatically or prompt a refresh.

Suggested Solution: Introduce a real-time event (WebSocket/SSE/push notification) such as TASKS_UPDATED or TASK_ASSIGNED. When a task is assigned, the backend emits the event to the affected user. Upon receiving the event, the frontend invalidates the cached task list and re-fetches the tasks.

[SagiEv]

The issue we're experiencing is caused by frontend caching, specifically due to how React Query is configured in your application.

Here is exactly what is happening:

1. The staleTime Configuration: In frontend/app/api/queries.ts, the queries for fetching tasks (like useMyTasks, useAvailableTasks, and useAdminTasks) are configured with a staleTime of 5 minutes (5 * 60 * 1000).
2. Freshness of Data: staleTime tells React Query how long the data should be considered "fresh". While the data is fresh (within those 5 minutes), React Query will serve the task list directly from the local cache in memory.
3. No Background Refetches: Because the data is considered fresh, navigating to the tasks screen, switching tabs, or focusing the window will not trigger a new network request to the backend.
4. Backend is Not Caching: Your backend (TaskController.java and TaskService.java) does not have any aggressive caching mechanisms (like @Cacheable or long Cache-Control headers) for these endpoints. If a request actually reached the backend, it would query the database and return the newly added task.

In summary: When you log in, the app fetches and caches the task list for 5 minutes. If a researcher adds a task during that window, your frontend won't even ask the backend for an updated list until those 5 minutes expire and the data becomes "stale".