Add kraken_minute table with 15-second polling

1-minute OHLC candles from Kraken, polled every 15 seconds to keep the
in-progress candle fresh. Unlike hourly/daily pollers, this one keeps
the last candle and upserts it as it updates.

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

resources/system.edn

@@ -63,6 +63,10 @@
  {:query-fn #ig/ref :db.sql/query-fn
   :enabled? #or [#env KRAKEN_ENABLED "true"]}
 
+ :kraken/ohlc-minute
+ {:query-fn #ig/ref :db.sql/query-fn
+  :enabled? #or [#env KRAKEN_ENABLED "true"]}
+
  :frankfurter/rates
  {:url      #or [#env FRANKFURTER_URL "http://localhost:8080"]
   :query-fn #ig/ref :db.sql/query-fn}