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/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 (