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/data/imarket.go

@@ -12,4 +12,9 @@ type IMarket interface {
 	// Alpha token methods
 	IsAlphaToken(symbol string) bool
 	GetAlphaToken(symbol string) (market.AlphaTokenInfo, bool)
+
+	// Trading pair methods
+	IsSpotPair(symbol string) bool
+	IsFuturesPair(symbol string) bool
+	RefreshTradingPairCache()
 }