nickserv: add SET (password/email) and DROP

SET PASSWORD changes your password (derived off-thread like register) and
SET EMAIL sets an address. DROP deletes your account, releases and drops
the channels you founded, and logs you out. A dropped account federates,
and each node logs out any local session that was relying on it.

modules/nickserv/drop.rs

@@ -0,0 +1,33 @@
+use crate::engine::db::Db;
+use crate::engine::service::{Sender, ServiceCtx};
+use crate::engine::state::Network;
+
+// DROP <password>: delete your account. Re-authenticates as confirmation, releases
+// and drops the channels you found, and logs you out.
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &Network, db: &mut Db) {
+    let Some(account) = from.account else {
+        ctx.notice(me, from.uid, "You need to be logged in. Identify to NickServ first.");
+        return;
+    };
+    let Some(&password) = args.get(1) else {
+        ctx.notice(me, from.uid, "Syntax: DROP <password>");
+        return;
+    };
+    if db.authenticate(account, password).is_none() {
+        ctx.notice(me, from.uid, "Invalid password.");
+        return;
+    }
+    let channels = db.channels_owned_by(account);
+    for chan in &channels {
+        let _ = db.drop_channel(chan);
+        ctx.channel_mode("", chan, "-r"); // server-sourced: release the registered mode
+    }
+    let _ = db.drop_account(account);
+    for uid in net.uids_logged_into(account) {
+        ctx.logout(&uid);
+    }
+    ctx.notice(me, from.uid, format!("Your account \x02{account}\x02 has been dropped."));
+    if !channels.is_empty() {
+        ctx.notice(me, from.uid, format!("Channels released: {}.", channels.join(", ")));
+    }
+}