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/topic.rs

@@ -1,8 +1,8 @@
-use crate::engine::db::Db;
+use crate::engine::db::Store;
 use crate::engine::service::{Sender, ServiceCtx};
 
 // TOPIC <#channel> <text>: set the channel topic (empty clears it).
-pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db: &Db) {
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db: &dyn Store) {
     let Some(&chan) = args.get(1) else {
         ctx.notice(me, from.uid, "Syntax: TOPIC <#channel> <text>");
         return;