**feat(webhook):** add WEBHOOK_SECRET for token authentication support

- Protect `/webhook` endpoint using the `Authorization` header
- Update `README.md` with setup instructions and examples for authentication
- Warn when `WEBHOOK_SECRET` is not configured
- Add tests for valid, missing, and invalid token scenarios
- Update `docker-compose.yml` to support `WEBHOOK_SECRET` configuration

pkg/diunwebhook/export_test.go

@@ -13,3 +13,7 @@ func ResetTags() {
 	db.Exec(`DELETE FROM tag_assignments`)
 	db.Exec(`DELETE FROM tags`)
 }
+
+func ResetWebhookSecret() {
+	SetWebhookSecret("")
+}