Add html_body field for multipart email notifications

Callers can now supply a plain-text Body alongside an HTML html_body;
the SMTP sender emits multipart/alternative using both verbatim instead
of auto-stripping HTML to derive the plain-text fallback. The legacy
content_type=HTML path is preserved (deprecated) for existing callers.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

api/grpc/handler.go

@@ -66,6 +66,7 @@ func (h *NotifierHandler) SendNotification(ctx context.Context, req *pb.SendNoti
 		Priority:    domain.Priority(req.Priority),
 		Subject:     req.Subject,
 		Body:        req.Body,
+		HTMLBody:    req.HtmlBody,
 		ContentType: contentType,
 		Recipients:  req.Recipients,
 		CC:          req.Cc,
@@ -368,6 +369,7 @@ func convertDomainToProtoNotification(notif *domain.Notification) *pb.Notificati
 		Status:     convertDomainToProtoStatus(notif.Status),
 		Subject:    notif.Subject,
 		Body:       notif.Body,
+		HtmlBody:   notif.HTMLBody,
 		Recipients: notif.Recipients,
 		Metadata:   convertInterfaceMapToString(notif.Metadata),
 		CreatedAt:  timestamppb.New(notif.CreatedAt),