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/imarket.go

@@ -17,5 +17,5 @@ type IMarket interface {
 	// Trading pair methods
 	IsSpotPair(symbol string) bool
 	IsFuturesPair(symbol string) bool
-	RefreshTradingPairCache()
+	RefreshTradingPairCache() error
 }