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>

resources/system.edn

@@ -56,10 +56,12 @@
   :enabled? #or [#env BINANCE_ENABLED "true"]}
 
  :kraken/ohlc
- {:query-fn #ig/ref :db.sql/query-fn}
+ {:query-fn #ig/ref :db.sql/query-fn
+  :enabled? #or [#env KRAKEN_ENABLED "true"]}
 
  :kraken/ohlc-day
- {:query-fn #ig/ref :db.sql/query-fn}
+ {:query-fn #ig/ref :db.sql/query-fn
+  :enabled? #or [#env KRAKEN_ENABLED "true"]}
 
  :frankfurter/rates
  {:url      #or [#env FRANKFURTER_URL "http://localhost:8080"]