refactored notificatoin / tgsettle responsibility boundaries

api/proto/notification_sent.proto

@@ -4,10 +4,17 @@ option go_package = "github.com/tech/sendico/pkg/generated/gmessaging";
 
 import "operation_result.proto";
 
+// NotificationSentEvent is published after a notification has been delivered
+// (or delivery has failed) to a user.
 message NotificationSentEvent {
+    // user_id identifies the recipient.
     string user_id = 1;
+    // template_id is the notification template that was rendered.
     string template_id = 2;
+    // channel is the delivery channel (e.g. "email", "sms", "push").
     string channel = 3;
+    // locale is the language/region used for rendering (e.g. "en", "ru").
     string locale = 4;
+    // status reports whether the delivery succeeded.
     OperationResult status = 5;
 }