Add real-time BTC price feed via Binance WebSocket

Connect to Binance trade stream, persist prices to binance_price table
(throttled to every 5s), and live-update the home page via HTMX polling.

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

resources/system.edn

@@ -51,4 +51,8 @@
 
  :reitit.routes/ui
  {:base-path ""
-  :query-fn  #ig/ref :db.sql/query-fn}}
+  :query-fn  #ig/ref :db.sql/query-fn}
+
+ :ws/binance
+ {:query-fn #ig/ref :db.sql/query-fn
+  :uri      "wss://stream.binance.com:9443/ws/btcusdt@trade"}}