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/configs/key/env.go

@@ -4,4 +4,5 @@ const (
 	LogEnv = "LOG_ENV"
 
 	TelegramToken = "TELEGRAM_TOKEN"
+	AdminChatID   = "ADMIN_CHAT_ID"
 )