refactor: verify 응답에서 userId 제거

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/auth/handler.go

@@ -89,13 +89,12 @@ func (h *Handler) VerifyToken(c *fiber.Ctx) error {
 		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "token 필드가 필요합니다"})
 	}
 
-	userID, username, err := h.svc.VerifyToken(req.Token)
+	username, err := h.svc.VerifyToken(req.Token)
 	if err != nil {
 		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": err.Error()})
 	}
 
 	return c.JSON(fiber.Map{
-		"userId":   userID,
 		"username": username,
 	})
 }