Add KRAKEN_ENABLED flag to disable Kraken pollers in local dev

Same pattern as BINANCE_ENABLED and STRIKE_ENABLED. Prevents local
dev from fetching external pricing data when pointed at btcprod.
Docker explicitly sets KRAKEN_ENABLED=true.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -19,6 +19,7 @@ services:
       CORS_ORIGIN: "${DOCKER_CORS_ORIGIN}"
       FRANKFURTER_URL: "${DOCKER_FRANKFURTER_URL}"
       STRIKE_API_KEY: "${STRIKE_API_KEY}"
+      KRAKEN_ENABLED: "true"
     extra_hosts:
       - "postgres:host-gateway"