Services no longer receive the concrete account store. on_command now takes
&mut dyn Store + &dyn NetView (both defined in fedserv-api); the engine's Db
and Network implement them. Reads hand back plain views (AccountView,
ChannelView, ...) carrying only non-secret fields, so the event log, gossip,
and credential material (password hash, SCRAM verifiers) are unreachable from
a module. All command modules ported; behaviour unchanged.
Mirrors the chanserv layout: nickserv.rs keeps the service and dispatch,
each command (register, identify, logout, cert) lives in its own file
with its subcommands and parameters. Behaviour is unchanged.
