fix: move Metabase credentials to runtime config

Move hardcoded Metabase credentials to Nuxt runtime config to
allow configuration via environment variables and follow security
best practices.

Changes:
- Update server/utils/metabase.ts to use useRuntimeConfig()
- Add metabase config to nuxt.config.ts runtimeConfig
- Update .env.example with Metabase configuration template

Environment variables:
- METABASE_URL (default: https://metabase.nucleoriofrio.com)
- METABASE_EMAIL (required for authentication)
- METABASE_PASSWORD (required for authentication)

.env.example

@@ -1,2 +1,7 @@
 # Authentik Configuration
 NUXT_PUBLIC_AUTHENTIK_URL=https://authentik.nucleoriofrio.com
+
+# Metabase Configuration (server-side only)
+METABASE_URL=https://metabase.nucleoriofrio.com
+METABASE_EMAIL=your-email@domain.com
+METABASE_PASSWORD=your-password