Build a social media feed

• Users can follow(userId, targetId) and unfollow; following yourself or an unknown user is rejected, and repeat calls are idempotent
• post(userId, text) creates a post with a unique id and a timestamp from an injectable clock
• getFeed(userId, limit) returns the most recent posts from the users they follow (optionally including their own), newest first, with ties broken by post id
• The feed is built by merging each followee's already time-ordered post list (a k-way merge using a max-heap), not by sorting every post in the system
• Pagination with a cursor (the last seen post id or timestamp) returns the next page without duplicates or gaps when new posts arrive
• Deleting a post removes it from every feed immediately

Only after every requirement works.

• Fan-out on write with cached feeds for users with few followers, and fan-out on read for celebrities (hybrid)
• Likes and comment counts with top-posts ranking
• Mute and block lists