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/slack.go

@@ -185,6 +185,12 @@ func (s slackConvertor) WorkflowJob(p *api.WorkflowJobPayload) (SlackPayload, er
 	return s.createPayload(text, nil), nil
 }
 
+func (s slackConvertor) WorkflowStep(p *api.WorkflowStepPayload) (SlackPayload, error) {
+	text, _ := getWorkflowStepPayloadInfo(p, SlackLinkFormatter, true)
+
+	return s.createPayload(text, nil), nil
+}
+
 // Push implements payloadConvertor Push method
 func (s slackConvertor) Push(p *api.PushPayload) (SlackPayload, error) {
 	// n new commits