fix(workflow): add missing created_by column to workflow tables

The base Model::create() auto-appends created_by when timestamps are
enabled, but workflow_instances and workflow_transition_log were missing
this column — causing PDOException on fine imposition.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/migrations/Phase_103_007_add_created_by_to_workflow_tables.php

+<?php
+declare(strict_types=1);
+
+return [
+    'up' => "
+ALTER TABLE workflow_instances ADD COLUMN created_by BIGINT UNSIGNED NULL AFTER completed_at;
+ALTER TABLE workflow_transition_log ADD COLUMN created_by BIGINT UNSIGNED NULL AFTER created_at
+",
+    'down' => "
+ALTER TABLE workflow_instances DROP COLUMN created_by;
+ALTER TABLE workflow_transition_log DROP COLUMN created_by
+",
+];