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/nickserv/set.rs

@@ -1,8 +1,8 @@
-use crate::engine::db::Db;
+use crate::engine::db::Store;
 use crate::engine::service::{Sender, ServiceCtx};
 
 // SET PASSWORD <newpassword> | SET EMAIL [address]: change your account settings.
-pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db: &mut Db) {
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db: &mut dyn Store) {
     let Some(account) = from.account else {
         ctx.notice(me, from.uid, "You need to be logged in. Identify to NickServ first.");
         return;