fix: only bump feed sort time for reposts from followed authors

[barrydeen]

Summary

• Engagement subscriptions fetch reposts (kind 6) from anyone referencing events in the feed, not just followed authors
• Previously any repost would bump the original note to the top of the feed using the repost timestamp, even from non-followed users
• Now only reposts from authors in the current author filter (followed users) affect feed sort ordering; non-followed reposters still update repost counts

Test plan

• Follow user A who posted a note, verify their note appears in feed at normal position
• Have a followed user repost the note — verify it bumps to the top of the feed
• Have a non-followed user repost the same note — verify it does NOT bump to the top, but repost count increments