feat: add Metabase API integration

Add complete integration with Metabase API to fetch data from
'facturador supabase' database.

Features:
- Server-side Metabase authentication using session tokens
- Utility functions for Metabase API requests with auto-retry
- API endpoints to proxy Metabase requests
  - GET /api/metabase/databases - List all databases
  - GET /api/metabase/tables/:databaseId - Get tables and metadata
  - POST /api/metabase/query - Execute queries against tables
- useMetabase() composable for frontend consumption
  - getDatabases() - Fetch available databases
  - getDatabaseMetadata() - Get tables and fields info
  - queryTable() - Execute queries with filters and limits
  - resultToObjects() - Helper to convert results to objects

Session tokens are cached and auto-refreshed when expired.
This enables the application to display real data from the
facturador database without using embeds or iframes.

nuxt4-app/server/api/metabase/databases.get.ts

@@ -0,0 +1,15 @@
+/**
+ * Get list of databases from Metabase
+ */
+export default defineEventHandler(async (event) => {
+  try {
+    const databases = await getMetabaseDatabases()
+    return databases
+  } catch (error: any) {
+    console.error('[API] Failed to get Metabase databases:', error)
+    throw createError({
+      statusCode: error.statusCode || 500,
+      statusMessage: error.statusMessage || 'Failed to fetch databases'
+    })
+  }
+})