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/tables/[databaseId].get.ts

@@ -0,0 +1,24 @@
+/**
+ * Get tables from a specific Metabase database
+ */
+export default defineEventHandler(async (event) => {
+  const databaseId = getRouterParam(event, 'databaseId')
+
+  if (!databaseId) {
+    throw createError({
+      statusCode: 400,
+      statusMessage: 'Database ID is required'
+    })
+  }
+
+  try {
+    const metadata = await getMetabaseTables(parseInt(databaseId))
+    return metadata
+  } catch (error: any) {
+    console.error(`[API] Failed to get tables for database ${databaseId}:`, error)
+    throw createError({
+      statusCode: error.statusCode || 500,
+      statusMessage: error.statusMessage || 'Failed to fetch tables'
+    })
+  }
+})