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/forms/repo_form.go

@@ -230,6 +230,7 @@ type WebhookForm struct {
 	Status                   bool
 	WorkflowRun              bool
 	WorkflowJob              bool
+	WorkflowStep             bool
 	Active                   bool
 	BranchFilter             string `binding:"GlobPattern"`
 	AuthorizationHeader      string