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/packagist.go

@@ -122,6 +122,10 @@ func (pc packagistConvertor) WorkflowJob(_ *api.WorkflowJobPayload) (PackagistPa
 	return PackagistPayload{}, nil
 }
 
+func (pc packagistConvertor) WorkflowStep(_ *api.WorkflowStepPayload) (PackagistPayload, error) {
+	return PackagistPayload{}, nil
+}
+
 func newPackagistRequest(_ context.Context, w *webhook_model.Webhook, t *webhook_model.HookTask) (*http.Request, []byte, error) {
 	meta := &PackagistMeta{}
 	if err := json.Unmarshal([]byte(w.Meta), meta); err != nil {