bug: since_id becomes invalid after unfollowing a user, causing clients to get stuck

[jukfiuune]

Description

After unfollowing a user who appears as the last tweet in the timeline, the client is unable to fetch new tweets and gets stuck in an infinite loop or receives a 403 error.

Steps to Reproduce

1. Log in and fetch the timeline - everything works normally
2. Unfollow a person whose tweet is currently the last one in the timeline
3. The client attempts to fetch new tweets using the saved since_id
4. The Bridge either returns a 403 (missing since_id in the database) or the client enters a loop trying to reach the end of the friends timeline

Expected Behavior

The client should be able to continue fetching new tweets normally after unfollowing someone, without the invalidated since_id causing problems