FEATURE(BACKEND):Add Recently Viewed Posts

[AYUSH-P-SINGH]

feat: Add Recently Viewed Posts Backend

Description

Currently, DevConnect AI does not track which posts a user has recently viewed. Implement backend support to maintain a "Recently Viewed Posts" history for authenticated users.

Whenever a user opens a post, the backend should update a recentlyViewed list in the user's Firestore document. The list should:

Store only post IDs.
Avoid duplicate entries.
Move an already viewed post to the top.
Keep only the latest 10 viewed posts.

[prasiddhi-105]

Hiii please assign this issue to me!!