11 handlers were calling repos/ports directly, bypassing the
application layer. Extracted into proper use cases:
feed: get_public_feed, get_user_feed, get_tag_feed, get_popular_tags
profile: get_user_profile (with follow check), list_users,
count_local_users, list_local_followers, list_local_following
federation_management: set_also_known_as
Also registers 9 previously undocumented handlers in OpenAPI modules.