OperServ: add FORBID (nick/channel/email registration bans)

FORBID ADD/DEL/LIST bans a NICK, CHAN, or EMAIL glob from being
registered — a network-wide policy that gossips like an AKILL and every
node enforces. Nick registration is blocked in pre_register_check (both
NickServ REGISTER and the account-registration relay), and channel
registration in ChanServ REGISTER. New Forbid store + events, mirroring
the akill subsystem.

src/grpc.rs

@@ -168,6 +168,8 @@ fn to_wire(entry: &LogEntry) -> Option<ReplicationEvent> {
         | Event::ChannelNoExpire { .. }
         | Event::AkillAdded { .. }
         | Event::AkillRemoved { .. }
+        | Event::ForbidAdded { .. }
+        | Event::ForbidRemoved { .. }
         | Event::AccountExpiryWarned { .. }
         | Event::ChannelExpiryWarned { .. }
         | Event::AccountOperNoteSet { .. }