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>

routers/web/repo/setting/webhook.go

@@ -187,6 +187,7 @@ func ParseHookEvent(form forms.WebhookForm) *webhook_module.HookEvent {
 			webhook_module.HookEventStatus:                   form.Status,
 			webhook_module.HookEventWorkflowRun:              form.WorkflowRun,
 			webhook_module.HookEventWorkflowJob:              form.WorkflowJob,
+			webhook_module.HookEventWorkflowStep:             form.WorkflowStep,
 		},
 		BranchFilter: form.BranchFilter,
 	}