fix: perform server-side rendering only when user-agent is a bot

[harshdoesdev]

We should perform SSR only when the user-agent is a bot, otherwise we can skip the SSR step for normal users. It reduces the server response time from 3.7s to 1.5s (tested on fastn.com served locally).

[amitu]

@harshdoesdev, write a test for this also somehow.

[amitu]

@harshdoesdev we have to make sure SSR happens when we are doing fastn build. Can you confirm that is the case?

[amitu]

@harshdoesdev can we write some test to ensure bots are getting ssr, and non bots are not?

[harshdoesdev]

@harshdoesdev can we write some test to ensure bots are getting ssr, and non bots are not?

@amitu I attempted to create a test case for this using fbt, but it resulted in a stack overflow error when using fastn serve. Therefore, for the time being, can we incorporate this test directly into the workflow and write a shell or Python script for it?