Address PR #15 round 2: fail-closed admin guard, sync init, error reporting

1. /refresh now fail-closed: rejects all if ADMIN_CHAT_ID unset or invalid
2. Initial pair cache fill is synchronous — bot waits before accepting queries
3. /refresh reports failure when API fetch fails instead of always saying success

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

internal/data/market/trading_pairs.go

@@ -7,20 +7,20 @@ import (
 	"github.com/rs/zerolog/log"
 )
 
-func (ms *MarketData) refreshTradingPairCache() {
+func (ms *MarketData) refreshTradingPairCache() error {
 	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
 	defer cancel()
 
 	spotInfo, err := ms.spotClient.NewExchangeInfoService().Do(ctx)
 	if err != nil {
 		log.Error().Err(err).Msg("Failed to fetch spot exchange info")
-		return
+		return err
 	}
 
 	futuresInfo, err := ms.futuresClient.NewExchangeInfoService().Do(ctx)
 	if err != nil {
 		log.Error().Err(err).Msg("Failed to fetch futures exchange info")
-		return
+		return err
 	}
 
 	ms.pairCacheMutex.Lock()
@@ -45,6 +45,7 @@ func (ms *MarketData) refreshTradingPairCache() {
 		Int("spot", len(ms.spotPairs)).
 		Int("futures", len(ms.futuresPairs)).
 		Msg("Trading pair cache refreshed")
+	return nil
 }
 
 func (ms *MarketData) pairCacheRefreshLoop() {
@@ -68,6 +69,6 @@ func (ms *MarketData) IsFuturesPair(symbol string) bool {
 	return ms.futuresPairs[symbol]
 }
 
-func (ms *MarketData) RefreshTradingPairCache() {
-	ms.refreshTradingPairCache()
+func (ms *MarketData) RefreshTradingPairCache() error {
+	return ms.refreshTradingPairCache()
 }