#5 feat: Add index instrument type support and improve error handling

- Add 'index' as a valid instrumentType in Symbol model and validation schemas - Create migration to update database CHECK constraint for instrument_type - Implement duplicate handling in symbol creation using findOrCreate - Add duplicate detection and logging in bulk candle creation - Improve error handling in getLatestCandle to return success response instead of error - Update README to reflect support for indices - Ensure all functionalities remain intact while enhancing robustness