Add Kraken hourly OHLC poller with database storage

Fetches Bitcoin hourly candles from the Kraken public REST API
and upserts them into a kraken_hour table on a 5-minute poll interval,
following the existing Integrant component pattern.

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

resources/system.edn

@@ -56,4 +56,8 @@
 
  :ws/binance
  {:query-fn #ig/ref :db.sql/query-fn
-  :uri      "wss://stream.binance.com:9443/ws/btcusdt@trade"}}
+  :uri      "wss://stream.binance.com:9443/ws/btcusdt@trade"}
+
+ :kraken/ohlc
+ {:query-fn    #ig/ref :db.sql/query-fn
+  :interval-ms 300000}}