feat(webhook): add workflow_step webhook events for step-level CI notifications

Adds HookEventWorkflowStep event type that fires on every step state
transition (queued -> in_progress -> completed). Follows the same
pattern as the existing workflow_job events.

- New WorkflowStepPayload struct with run/job/step context
- WorkflowStepStatusUpdate notifier interface + dispatch
- Step state change detection in UpdateTask runner endpoint
- Fix: register workflow_step in updateHookEvents API mapping
- Full test coverage mirroring workflow_job tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/webhook/dingtalk.go

@@ -188,6 +188,12 @@ func (dingtalkConvertor) WorkflowJob(p *api.WorkflowJobPayload) (DingtalkPayload
 	return createDingtalkPayload(text, text, "Workflow Job", p.WorkflowJob.HTMLURL), nil
 }
 
+func (dingtalkConvertor) WorkflowStep(p *api.WorkflowStepPayload) (DingtalkPayload, error) {
+	text, _ := getWorkflowStepPayloadInfo(p, noneLinkFormatter, true)
+
+	return createDingtalkPayload(text, text, "Workflow Step", p.WorkflowJob.HTMLURL), nil
+}
+
 func createDingtalkPayload(title, text, singleTitle, singleURL string) DingtalkPayload {
 	return DingtalkPayload{
 		MsgType: "actionCard",