modules: capability-scoped Store/NetView boundary (Tier 2)

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.

modules/chanserv/seen.rs

@@ -1,8 +1,8 @@
 use crate::engine::service::{Sender, ServiceCtx};
-use crate::engine::state::Network;
+use crate::engine::state::NetView;
 
 // SEEN <nick>: when a nick was last seen, and doing what.
-pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &Network) {
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &dyn NetView) {
     let Some(&nick) = args.get(1) else {
         ctx.notice(me, from.uid, "Syntax: SEEN <nick>");
         return;