Handle SMTP 550 recipient-rejected errors with structured SmtpSendException

- Add SmtpSendException with smtpCode/smtpResponse/isRecipientRejected()
- smtpSend() $expect closure throws SmtpSendException (with code) instead of RuntimeException
- SmtpRelay::send() re-throws SmtpSendException so callers can inspect it
- request-access.php (new): catch 550 → roll back token+approval, return HTTP 422 with FR user message
- request-access.php (resend): catch 550 → HTTP 422 instead of silently claiming success
- StudentEmail::sendConfirmation(): catch SmtpSendException → log+false (submission not aborted)
- admin/actions/access-request.php: catch SmtpSendException post-approval → flash warning (recipient-rejected vs transient)

app/src/StudentEmail.php

@@ -99,7 +99,13 @@ class StudentEmail {
         $subject   = 'Merci — ton TFE a bien été enregistré';
         $htmlBody  = self::buildHtml($thesis);
 
-        $result = SmtpRelay::send($db, $to, $subject, $htmlBody);
+        try {
+            $result = SmtpRelay::send($db, $to, $subject, $htmlBody);
+        } catch (SmtpSendException $e) {
+            // Confirmation email failure must not abort the successful submission.
+            error_log("[StudentEmail] SMTP error sending to {$to} for thesis #{$thesisId}: " . $e->getMessage());
+            return false;
+        }
 
         if ($result) {
             error_log("[StudentEmail] Confirmation sent to {$to} for thesis #{$thesisId}");