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>

templates/repo/settings/webhook/settings.tmpl

@@ -346,6 +346,16 @@
 				</div>
 			</div>
 		</div>
+		<!-- Workflow Step Event -->
+		<div class="seven wide column">
+			<div class="field">
+				<div class="ui checkbox">
+					<input name="workflow_step" type="checkbox" {{if .Webhook.HookEvents.Get "workflow_step"}}checked{{end}}>
+					<label>{{ctx.Locale.Tr "repo.settings.event_workflow_step"}}</label>
+					<span class="help">{{ctx.Locale.Tr "repo.settings.event_workflow_step_desc"}}</span>
+				</div>
+			</div>
+		</div>
 	</div>
 </div>