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/models/webhook_event.go

@@ -52,6 +52,20 @@ func (w *WebhookEvent) Reload(ctx context.Context) error {
 	return w.scan(row)
 }
 
+// ShortCommit returns a truncated commit SHA for display.
+func (w *WebhookEvent) ShortCommit() string {
+	if !w.CommitSHA.Valid {
+		return ""
+	}
+
+	sha := w.CommitSHA.String
+	if len(sha) > shortCommitLength {
+		return sha[:shortCommitLength]
+	}
+
+	return sha
+}
+
 func (w *WebhookEvent) insert(ctx context.Context) error {
 	query := `
 		INSERT INTO webhook_events (