Please consider that we're not all English-speaking, and that such terms may be unknown to people who aren't from your culture, even if we do understand your language. CCTV could mean "China Central TeleVision" for instance ;-)
In the context of surveillance cameras it is perfectly clear what CCTV stands for, and if it is an unknown to someone because they are not familiar with the english language it is also perfectly reasonable to just force them to look it up like they would any other english word they are unfamiliar with.
Acronyms are not the same as the English language as they are not words by themselves but compressions. "Closed-circuit television" is self-evident to a reader; CCTV isn't. And "in the context", yes, but readers are not necessarily experts in their fields. This is why many news publications usually expand acronyms.
So to be clear, I think that it would make sense for Frigate to define NVR the first time they use it on their site. However, this isn't a news publication and I really don't think it's unreasonable to expect any serious visitor to the Frigate site to be expert enough to know what an NVR is.
