email: outbound mail subsystem and RESETPASS

Adds a config-driven mailer: a SendEmail action the link layer pipes to
a configured command (sendmail -t, msmtp, ...) off-thread. NickServ
RESETPASS <account> emails a short-lived code to the address on file;
RESETPASS <account> <code> <newpassword> completes the reset. Codes are
node-local; the password change itself federates.

modules/nickserv/resetpass.rs

@@ -0,0 +1,42 @@
+use crate::engine::db::Db;
+use crate::engine::service::{Sender, ServiceCtx};
+
+// RESETPASS <account>: email a reset code to the address on file.
+// RESETPASS <account> <code> <newpassword>: complete the reset with that code.
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db: &mut Db) {
+    if !db.email_enabled() {
+        ctx.notice(me, from.uid, "Password reset by email isn't available here.");
+        return;
+    }
+    match (args.get(1), args.get(2), args.get(3)) {
+        (Some(&name), None, None) => {
+            let Some(canonical) = db.resolve_account(name).map(str::to_string) else {
+                ctx.notice(me, from.uid, format!("\x02{name}\x02 isn't registered."));
+                return;
+            };
+            let Some(email) = db.account(&canonical).and_then(|a| a.email.clone()) else {
+                ctx.notice(me, from.uid, "That account has no email on file, so it can't be reset.");
+                return;
+            };
+            let code = db.issue_reset_code(&canonical);
+            ctx.send_email(
+                email,
+                format!("Password reset for {canonical}"),
+                format!("Your password reset code for {canonical} is: {code}\nIt expires in 15 minutes. Reset with:\n  /msg NickServ RESETPASS {canonical} {code} <newpassword>"),
+            );
+            ctx.notice(me, from.uid, format!("A reset code has been emailed to the address on file for \x02{canonical}\x02."));
+        }
+        (Some(&name), Some(&code), Some(&newpass)) => {
+            let Some(canonical) = db.resolve_account(name).map(str::to_string) else {
+                ctx.notice(me, from.uid, format!("\x02{name}\x02 isn't registered."));
+                return;
+            };
+            if !db.take_reset_code(&canonical, code) {
+                ctx.notice(me, from.uid, "Invalid or expired reset code.");
+                return;
+            }
+            ctx.defer_password(&canonical, newpass, me, from.uid);
+        }
+        _ => ctx.notice(me, from.uid, "Syntax: RESETPASS <account> [<code> <newpassword>]"),
+    }
+}