feat: add webhook event history UI page

Add a per-app webhook event history page at /apps/{id}/webhooks that
displays received webhook events with their match/no-match status,
event type, branch, commit SHA (linked to commit URL when available),
and timestamp.

Changes:
- Add webhook_events.html template following existing UI patterns
- Add HandleAppWebhookEvents handler in webhook_events.go
- Register GET /apps/{id}/webhooks route
- Register webhook_events.html in template cache
- Add ShortCommit() method to WebhookEvent model
- Add 'Event History' link to app detail Webhook URL section

templates/templates.go

@@ -44,6 +44,7 @@ func initTemplates() {
 		"app_detail.html",
 		"app_edit.html",
 		"deployments.html",
+		"webhook_events.html",
 	}
 
 	pageTemplates = make(map[string]*template.Template)