Addresses review concern: the fuer-dich predicate (youMentioned ||
youParticipated) had zero test coverage after feedFilters.test.ts was
deleted. The new clientFilter module is a pure function that is directly
testable, and the test explicitly documents why MENTION_CREATED items
without the youMentioned flag are now excluded (they would have shown
mentions directed at OTHER users under the old feedFilters.ts logic).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>