Address PR #15 review: batch API calls and admin-guard /refresh

1. Add GetAllPremiumIndex() to fetch all futures data in one call,
   used by GetTopPrices instead of per-symbol sequential calls.
2. Add ADMIN_CHAT_ID env check to /refresh command to restrict
   access to authorized users only.

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

internal/data/imarket.go

@@ -4,6 +4,7 @@ import "me.thuanle/bbot/internal/data/market"
 
 type IMarket interface {
 	GetFuturePrice(symbol string) (float64, float64, int64, bool)
+	GetAllPremiumIndex() (map[string]market.PremiumIndex, error)
 	GetAllFundRate() (map[string]float64, map[string]int64)
 
 	GetSpotPrice(symbol string) (float64, bool)