Limit minute poller to last 5 candles after initial fetch

Reduces unnecessary upserts on each 15-second poll cycle by only
processing the 5 most recent candles from Kraken's response.

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

src/clj/pmagnus/btcdata/kraken/ohlc_minute.clj

@@ -55,13 +55,14 @@
 
 (defn- poll!
   "Fetch OHLC data including the last in-progress candle, upsert all.
+   On initial fetch saves everything; subsequent fetches only take last 5.
    Returns the count of saved candles."
-  [client query-fn since-atom]
+  [client query-fn since-atom initial?]
   (let [result   (fetch-ohlc client @since-atom)
         last-ts  (:last result)
         pair-key (first (remove #{:last} (keys result)))
         raw      (get result pair-key)
-        candles  (map parse-candle raw)]
+        candles  (map parse-candle (if initial? raw (take-last 5 raw)))]
     (when (seq candles)
       (save-candles! query-fn candles)
       (when last-ts
@@ -74,14 +75,14 @@
   [client query-fn since-atom running?]
   (future
     (try
-      (poll! client query-fn since-atom)
+      (poll! client query-fn since-atom true)
       (catch Exception e
         (log/error e "Kraken minute initial poll failed")))
     (while @running?
       (Thread/sleep 15000)
       (when @running?
         (try
-          (poll! client query-fn since-atom)
+          (poll! client query-fn since-atom false)
           (catch Exception e
             (log/error e "Kraken minute poll failed")))))))