[API] Properly handle non-BigInt authorID's

There should be some proper error handling for this. I initially did it like this because I was lazy, and once again to get the actual bot and API working.

The way it currently does it works, but it's quite hard to understand. It should at least specify that it needs a BigInt and not a random string instead of doing what it currently does. https://github.com/spifory/tags/blob/7bc95abf9b74e332604aceb4766dccb4e53db2e9/api/src/previews/previewService.ts#L12-L15