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/payloader.go

@@ -31,6 +31,7 @@ type payloadConvertor[T any] interface {
 	Status(*api.CommitStatusPayload) (T, error)
 	WorkflowRun(*api.WorkflowRunPayload) (T, error)
 	WorkflowJob(*api.WorkflowJobPayload) (T, error)
+	WorkflowStep(*api.WorkflowStepPayload) (T, error)
 }
 
 func convertUnmarshalledJSON[T, P any](convert func(P) (T, error), data []byte) (t T, err error) {
@@ -86,6 +87,8 @@ func newPayload[T any](rc payloadConvertor[T], data []byte, event webhook_module
 		return convertUnmarshalledJSON(rc.WorkflowRun, data)
 	case webhook_module.HookEventWorkflowJob:
 		return convertUnmarshalledJSON(rc.WorkflowJob, data)
+	case webhook_module.HookEventWorkflowStep:
+		return convertUnmarshalledJSON(rc.WorkflowStep, data)
 	}
 	return t, fmt.Errorf("newPayload unsupported event: %s", event)
 }