graph builder refactor for maintainability

[jahndan]

"a little functional programming never hurt anybody" but it's also much harder to onboard/maintain when it's incomprehensible

so basically this is a reminder that i should eventually rewrite graph builder using OOP instead for maintainability, although this is a fairly low priority right now as graph builder is currently working (and I'll be here to maintain it for a while) — because eventually someone else will have to deal with that code

also right now it takes multiple root courses and efficiently builds the full graph from them, but I want to rewrite (or at least, supplement) it in a way that allows for caching of graphs built from individual root courses, and then combining them after the fact - this should also play well with any serverless/edge functions we might want to use for this instead (if I maybe want to use python (wasm? 💀) for set-based graph building, once we get accurate and stabilized data)