`get_existing_candles` times out on larger PostgreSQL databases because of N+1 queries

The get_existing_candles() endpoint can exceed the MCP client's 10-second request timeout when the Jesse candle database contains multiple exchange/symbol datasets.

Observed error:

Network error during existing candles retrieval:
HTTPConnectionPool(host='127.0.0.1', port=9000):
Read timed out. (read timeout=10)