Trim `backend` dependencies

[onkoe]

Currently, we're sitting at a whopping 400 dependencies on the backend!

It'd be nice to trim that down to something more manageable.

Checklist

• Remove all procedural macros (from serde, rocket, etc.)
• Try moving to a more minimal crate than webrtc
  • This mf takes up literally ~250/400 of those dependencies lmao