Nice! 
Discussion
it's not copying the new URL. Only taking the tenor one. It says it is copied but isn't maybe you need a prompt for brave to ask permissions to use the clipboard? Brave on Linux Mint. If it showed the full url in the pop up it could also manually be copied.
The copied link is a Tenor link by intent
When you click a gif, a separate API call is made on the back end to upload to nostr.build
That process takes time that would really hinder the user experience if you had to wait for the new url to be generated in order copy it; it’s several seconds
I could potentially make a separate API that will return URL instantly, and will redirect to Tenor while upload is prepared, and then serve local copy. If you are interested, let me know.
There have been a couple comments in my feed from people expecting to paste a nostr.build address so that could be cool
Your API is fast already, it's mostly that nip98 AUTH requires me to publish a Kind 27235 note, which takes time to broadcast with my current library
Were you thinking this API would be separate from nip98?
Nip98 does not require anything, just a signed header. You are talking about nip94, that can be pushed async.
I would probably need that for user uploaded gifs actually now that I think about it more