Backend: Add backend logic for filtering posts by unread, updated, and unanswered statuses

[cindy-wg]

Task: Add backend logic for unread, updated, and unanswered status by tracking user interactions and post updates in the database to support filter options.
Estimated Effort: High
Dependencies: #25
Acceptance Criteria: Backend database supports filtering by unread, updated, and unanswered posts and calling for a status correctly filters posts by it.

Milestone Update: Could not get to this because of work spent on search feature backend.