feat: add JSON and Excel export alongside CSV in SQL editor

[aalvaropc]

Summary

• Add elixlsx dependency for Excel (.xlsx) generation
• New export actions: POST /exports/json (JSON array of objects) and POST /exports/xlsx (Excel workbook)
• Export button in SQL editor toolbar replaced with dropdown menu: CSV, JSON, Excel
• Refactored ExportController with with_result/4 helper to eliminate code duplication
• JSON export maps each row to {column: value} objects
• Excel export handles Decimal, boolean, nil, and numeric types as proper cell values

Test plan

• mix precommit — 638 tests, 0 failures
• Manual: run a query, export as JSON — verify valid JSON array; export as Excel — verify .xlsx opens correctly