feat: add webhook event history UI page (#164)

## Summary

Adds a per-app webhook event history page at `/apps/{id}/webhooks` showing received webhook events with match/no-match status.

## Changes

- **New template** `webhook_events.html` — displays webhook events in a table with time, event type, branch, commit SHA (linked when URL available), and match status badges
- **New handler** `HandleAppWebhookEvents()` in `webhook_events.go` — fetches app and its webhook events (limit 100)
- **New route** `GET /apps/{id}/webhooks` — registered in protected routes group
- **Template registration** — added `webhook_events.html` to the template cache in `templates.go`
- **Model enhancement** — added `ShortCommit()` method to `WebhookEvent` for truncated SHA display
- **App detail link** — added "Event History" link in the Webhook URL card on the app detail page

## UI

Follows the existing UI patterns (Tailwind CSS classes, Alpine.js `relativeTime`, badge styles, empty state, back-navigation). The page mirrors the deployments history page layout.

closes [#85](#85)

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #164
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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())