refactor: make refund metrics API URL configurable

- Add refundMetricsApiUrl to Docusaurus customFields
- Allows easy configuration without environment variables
- Consistent with BuilderNet implementation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: odysseus0

docusaurus.config.js

@@ -120,5 +120,8 @@ module.exports = async function createConfigAsync() {
       },
       'docusaurus-plugin-sass'
     ],
+    customFields: {
+      refundMetricsApiUrl: 'https://refund-metrics-dune-api.vercel.app',
+    },
   }
 }