Align backend plan with codebase

lib/server/mail.ts

@@ -0,0 +1,24 @@
+import nodemailer from "nodemailer";
+import { logger } from "../logger";
+
+export async function sendOtpEmail(to: string, code: string): Promise<void> {
+  const url = process.env.SMTP_URL;
+
+  if (!url) {
+    if (process.env.NODE_ENV === "development") {
+      logger.info(`[dev] OTP for ${to}: ${code}`);
+      return;
+    }
+    throw new Error("SMTP_URL is not configured");
+  }
+
+  const transporter = nodemailer.createTransport(url);
+  const from = process.env.SMTP_FROM ?? "noreply@localhost";
+
+  await transporter.sendMail({
+    from,
+    to,
+    subject: "Your Community Rule sign-in code",
+    text: `Your sign-in code is: ${code}\n\nIt expires in 10 minutes.`,
+  });
+}