I'd say that there probably is a risk, and that using fresh lemon juice may increase that depending on where in the process you add it, but you'll have some microbiological risk anyway - the brewing process *might* address the relevant parts of the microbiological loading of incoming ingredients and the equipment you're using, but it also might not, but in any case the overall brewing and filling process itself can also have an impact.
If the brewing process temperature and time are sufficient then in effect this could pasteurise the product including the juice component you're adding, but if the lemon is added as a second stage once cooled then the freshly squeezed juice is almost definitely going to increase the potential for microbiological spoilage.
Depending on the pH, water activity etc of the finished product there may also be a risk of more serious microbiological issues that could make you ill (or possibly worse, in an extreme case), although not specifically due to the lemon.
What are you planning to pack it into and how are you planning to store it?
I've seen industrially supplied tea "bases" that are used for bottled iced tea type products with up to 1 month ambient / 6 month chilled shelf life, but these are pH controlled, have a known microbiological status as a result of being produced through a validated process, and are generally also high Brix (upper 60s / low 70s) and/or contain added preservative.
If I was doing this at home I'd probably buy a cheap pH meter and use that to make sure that I kept the pH as low as I could without adversely affecting the flavour - without having looked at any proper data for this type of product I'd aim for <4 if possible. I'd probably store it frozen too, as that tends to stop most things growing