Rewrite price lookup from WebSocket to REST API

Replace unreliable WebSocket connections with on-demand REST API calls
for spot and futures prices. Add cached trading pair list (refreshed
hourly) for symbol validation, and /refresh command for manual updates.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/helper/binancex/symbol.go

@@ -29,8 +29,7 @@ var (
 )
 
 func testSym(sym string) bool {
-	_, _, _, test := data.Market.GetFuturePrice(sym)
-	return test
+	return data.Market.IsFuturesPair(sym)
 }
 
 func Token2Symbols(token string) []string {