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/main.go

@@ -36,6 +36,9 @@ func NewMarketData() *MarketData {
 		futuresClient: futures.NewClient("", ""),
 	}
 
+	if err := ms.refreshTradingPairCache(); err != nil {
+		log.Error().Err(err).Msg("Failed initial trading pair cache load")
+	}
 	go ms.pairCacheRefreshLoop()
 	go ms.alphaCacheRefreshLoop()