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

internal/server/routes.go

@@ -70,6 +70,7 @@ func (s *Server) SetupRoutes() {
 			r.Post("/apps/{id}/deploy", s.handlers.HandleAppDeploy())
 			r.Post("/apps/{id}/deployments/cancel", s.handlers.HandleCancelDeploy())
 			r.Get("/apps/{id}/deployments", s.handlers.HandleAppDeployments())
+			r.Get("/apps/{id}/webhooks", s.handlers.HandleAppWebhookEvents())
 			r.Get("/apps/{id}/deployments/{deploymentID}/logs", s.handlers.HandleDeploymentLogsAPI())
 			r.Get("/apps/{id}/deployments/{deploymentID}/download", s.handlers.HandleDeploymentLogDownload())
 			r.Get("/apps/{id}/logs", s.handlers.HandleAppLogs())