Feat: Implement GET `v1/tasks/watchlist/` API to list tasks in user's watchlist

[Achintya-Chatterjee]

Issue Description

Implement a RESTful API endpoint to retrieve a list of all tasks that an authenticated user has added to their personal watchlist. This supports the "Watchlist" USP feature and its dedicated tab.

• PRD: https://docs.google.com/document/d/1KOFK8ISE1hy9h8QjNbrObo6QKeM1I-rYZTJ8ygwfxhg/edit?usp=sharing

Expected Behaviour

When the API is called by an authenticated user:

1. It should return a list of all tasks currently in that user's watchlist.
2. The list should be paginated, similar to the main /tasks/ endpoint.
3. It should support sorting options relevant to a watchlist (e.g., by priority, due date, date added to watchlist). PRD: "Watchlist items should be prioritised for easier tracking."
4. The API should return a 200 OK status on success.
5. If the watchlist is empty, it should return an empty list with a 200 OK status.

Current Behaviour

Currently:

1. There is no dedicated endpoint to list tasks in a user's watchlist.
2. Watchlisted tasks (once implemented) would not be easily viewable in a separate list for the dedicated tab.

Screenshots

N/A

Reproducibility

• This issue is reproducible
• This issue is not reproducible

Steps to Reproduce

1. Add one or more tasks to the watchlist (once the "add to watchlist" API is implemented).
2. Attempt to GET `v1/tasks/watchlist/`.
3. Observe that the endpoint does not exist, and there's no dedicated way to list watchlisted tasks.

Severity/Priority

• Critical
• High (Supports a core USP Feature's dedicated view)
• Medium
• Low

Additional Information

• This endpoint will query tasks based on their association with the authenticated user's watchlist.
• The task data returned should be consistent with other task-listing endpoints.

Checklist

• I have read and followed the project's code of conduct.
• I have searched for similar issues before creating this one.
• I have provided all the necessary information to understand and reproduce the issue.
• I am willing to contribute to the resolution of this issue.