Finish MemoServ on the typed, event-logged memo model: Db memo_send/list/read/
del/unread_memos + Store facade, and a new fedserv-memoserv crate (a default
service). Beyond the basics: SEND caps a mailbox (30) so nobody's flooded, READ
takes <num>|NEW|ALL, DEL takes <num>|ALL, LIST previews and flags unread, and
NickServ notifies you of waiting memos on login. Memos persist and federate
(Global scope) like other account data. Also lands NetAction::QuitUser (BotServ
plumbing). Data + delivery/login-notify tests.
