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/convert/convert.go

@@ -357,6 +357,20 @@ func ToActionsStatus(status actions_model.Status) (string, string) {
 	return action, conclusion
 }
 
+// ToActionWorkflowStep converts an actions_model.ActionTaskStep to an api.ActionWorkflowStep.
+// The step index (0-based position) must be passed by the caller because it is not stored on the model.
+func ToActionWorkflowStep(step *actions_model.ActionTaskStep, stepIndex int) *api.ActionWorkflowStep {
+	stepStatus, stepConclusion := ToActionsStatus(step.Status)
+	return &api.ActionWorkflowStep{
+		Name:        step.Name,
+		Number:      int64(stepIndex),
+		Status:      stepStatus,
+		Conclusion:  stepConclusion,
+		StartedAt:   step.Started.AsTime().UTC(),
+		CompletedAt: step.Stopped.AsTime().UTC(),
+	}
+}
+
 // ToActionWorkflowJob convert a actions_model.ActionRunJob to an api.ActionWorkflowJob
 // task is optional and can be nil
 func ToActionWorkflowJob(ctx context.Context, repo *repo_model.Repository, task *actions_model.ActionTask, job *actions_model.ActionRunJob) (*api.ActionWorkflowJob, error) {