Make sats EUR amount configurable, default 54.46

Configurable via SATS_EUR_AMOUNT env var.

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

src/clj/pmagnus/btcprice/web/routes/ws_proxy.clj

@@ -117,9 +117,12 @@
         amount))
     "\u2014"))
 
+(def ^:private sats-eur-amount
+  (double (bigdec (or (System/getenv "SATS_EUR_AMOUNT") "54.46"))))
+
 (defn- eur-to-sats [rates]
   (when-let [eur-btc (get rates "EUR/BTC")]
-    (long (* 54 (double (bigdec eur-btc)) 100000000))))
+    (long (* sats-eur-amount (double (bigdec eur-btc)) 100000000))))
 
 (defn- strike-json->html [text]
   (let [rates (json/read-str text)