Move feed, channels, sync, playlists, tags, notifications, saved-views and
quota out of the lib/api.ts god object into per-domain slice modules, each
owning its own types + method slice and spread into the façade. The `api`
object shape and all 54 call sites are unchanged; account/auth/setup/admin
stay inline until S2.
Reunites the interleaved tags read/CRUD that motivated the epic.
