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/group.rs

@@ -1,9 +1,9 @@
-use crate::engine::db::Db;
+use crate::engine::db::Store;
 use crate::engine::service::{Sender, ServiceCtx};
 
 // GROUP <account> <password>: link your current nick to an existing account, so
 // you can identify to it under this nick too.
-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), Some(&password)) = (args.get(1), args.get(2)) else {
         ctx.notice(me, from.uid, "Syntax: GROUP <account> <password>");
         return;