Implement Kraken integration with REST and WebSocket clients, add market data handling, and enhance settings configuration

.env.example

@@ -7,3 +7,11 @@ DUCKDB_PATH=./data/arbitrade.duckdb
 FERNET_KEY=
 KRAKEN_API_KEY=
 KRAKEN_API_SECRET=
+KRAKEN_REST_URL=https://api.kraken.com
+KRAKEN_WS_URL=wss://ws.kraken.com/v2
+KRAKEN_PRIVATE_RATE_LIMIT_SECONDS=1.0
+KRAKEN_HTTP_TIMEOUT_SECONDS=10.0
+KRAKEN_RETRY_ATTEMPTS=3
+KRAKEN_RETRY_BASE_DELAY_SECONDS=0.25
+WS_HEARTBEAT_TIMEOUT_SECONDS=20.0
+WS_MAX_STALENESS_SECONDS=5.0