Log store write failures at the source; share the oper guard

The store's map_err(|_| Internal) discarded the real IO error, so a
disk-full or permissions failure was invisible; log it once in the log
append path where it happens. Add echo_api::require_oper (parameterised
by the required privilege) and route the three copied per-service guards
through it, keeping each service's own wording and policy.

modules/hostserv/src/lib.rs

@@ -103,11 +103,7 @@ impl Service for HostServ {
 
 // Operator gate for vhost administration.
 fn require_oper(me: &str, from: &Sender, ctx: &mut ServiceCtx) -> bool {
-    if from.privs.has(Priv::Admin) {
-        return true;
-    }
-    ctx.notice(me, from.uid, "Access denied — assigning vhosts is for services operators.");
-    false
+    echo_api::require_oper(me, from, ctx, Some(Priv::Admin), "assigning vhosts")
 }
 
 // Canonicalise a vhost the way the ircd will actually display it: the host part