refactor: migrate web server to axum (lite)

[robjtede]

blocked on #226

a minimal viable "migration" to axum

pros:

• –6 LoC
• smallest diff out of our options

cons:

• not using the axum router, still delegating to route-recognizer

tokei:

 Language            Files        Lines         Code     Comments       Blanks
===============================================================================
 Rust                   30         3946         3403           22          521
 |- Markdown             6           48            0           43            5
 (Total)                           3994         3403           65          526