add tag migrations and update send / saveEvent

db/mysql/migrations/07_add_tag/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE `website_event` ADD COLUMN `tag` VARCHAR(50) NULL;
+
+-- CreateIndex
+CREATE INDEX `website_event_website_id_created_at_tag_idx` ON `website_event`(`website_id`, `created_at`, `tag`);