feat: add optional plain-text fallback to sendEmail

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/email/index.ts

@@ -38,6 +38,7 @@ export async function sendEmail(
   subject: string,
   html: string,
   from?: string,
+  text?: string,
 ): Promise<nodemailer.SentMessageInfo> {
   const transporter = createTransporter();
 
@@ -46,12 +47,10 @@ export async function sendEmail(
     to: Array.isArray(to) ? to.join(', ') : to,
     subject,
     html,
+    ...(text ? { text } : {}),
   });
 
-  logger.debug(
-    { messageId: info.messageId, to, subject },
-    'Email sent',
-  );
+  logger.debug({ messageId: info.messageId, to, subject }, 'Email sent');
 
   return info;
 }