chanserv: op, deop, voice, devoice, kick, ban, unban

Adds channel moderation commands sourced from the ChanServ pseudoclient.
Tracks connected users' nicks and hosts so targets can be resolved by
nick and banned by *!*@host. All commands require operator access.

modules/chanserv/unban.rs

@@ -0,0 +1,22 @@
+use crate::engine::db::Db;
+use crate::engine::service::{Sender, ServiceCtx};
+use crate::engine::state::Network;
+
+// UNBAN <#channel> [nick]: remove the *!*@host ban of a user (self if no nick).
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &Network, db: &Db) {
+    let Some(&chan) = args.get(1) else {
+        ctx.notice(me, from.uid, "Syntax: UNBAN <#channel> [nick]");
+        return;
+    };
+    if !super::require_op(me, from, chan, ctx, db) {
+        return;
+    }
+    let target = args
+        .get(2)
+        .and_then(|&n| net.uid_by_nick(n))
+        .map(str::to_string)
+        .unwrap_or_else(|| from.uid.to_string());
+    let host = net.host_of(&target).unwrap_or("*");
+    ctx.channel_mode(me, chan, &format!("-b *!*@{host}"));
+    ctx.notice(me, from.uid, format!("Cleared the *!*@{host} ban on \x02{chan}\x02."));
+}