Remove blocking code and spawn_blocking

Rayhunter uses a mixture of spawn and spawn_blocking, then also does
some blocking operations inside of async code.

Move everything to async. This allows us to use the single-threaded
runtime.

Now the binary is 100kB smaller, and the memory usage also improved by
~100kB on tplink.

Cargo.lock

@@ -2422,6 +2422,7 @@ name = "rayhunter-daemon"
 version = "0.5.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "async_zip",
  "axum",
  "chrono",