chore(email): log successful sends for observability

Previously sendEmail() only logged on error/suppression paths; success
was silent. After prod deploy of c71c401 it was unclear whether the
first alert tick had actually delivered email or not - the answer was
yes but had to be derived from 'no error log + recipient query showed
matching device'. Add a log line on success so future observability
doesn't require detective work.

server/services/email.js

@@ -138,6 +138,7 @@ async function sendEmail({ to, subject, text, html }) {
   try {
     const token = await getAccessToken();
     await postSendMail(token, buildSendMailPayload(to, subject, text, html));
+    console.log(`[EMAIL] sent to ${to}: ${subject}`);
     return { sent: true };
   } catch (e) {
     console.error(`[EMAIL] Graph send failed for ${to}: ${e.message}`);