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/list.rs

@@ -1,9 +1,9 @@
-use crate::engine::db::Db;
+use crate::engine::db::Store;
 use crate::engine::service::{Sender, ServiceCtx};
 
 // LIST: show all registered channels.
-pub fn handle(me: &str, from: &Sender, _args: &[&str], ctx: &mut ServiceCtx, db: &Db) {
-    let mut names: Vec<&str> = db.channels().map(|c| c.name.as_str()).collect();
+pub fn handle(me: &str, from: &Sender, _args: &[&str], ctx: &mut ServiceCtx, db: &dyn Store) {
+    let mut names: Vec<String> = db.channels().into_iter().map(|c| c.name).collect();
     if names.is_empty() {
         ctx.notice(me, from.uid, "No channels are registered.");
         return;