Expose severity to display

See https://github.com/EFForg/rayhunter/issues/334

Severity levels low, medium, high are now exposed to the UI in form of
dotted, dashed and solid lines. The line on the UI represents the
highest-so-far severity seen.

Originally this was intended to be represented by Yellow/Orange/Red, but
this would mean yet another divergence for colorblind mode. This is
colorblind-friendly by default (I think...)

As part of this, simplify EventType so that it becomes a flat "level"
enum without nested variants.

There is also a new debug endpoint that allows one to overwrite the
display level directly for testing.

lib/src/util.rs

@@ -1,10 +1,10 @@
-use serde::Serialize;
+use serde::{Deserialize, Serialize};
 
 #[cfg(target_family = "unix")]
 use nix::sys::utsname::uname;
 
 /// Expose binary and system information.
-#[derive(Serialize, Debug)]
+#[derive(Serialize, Deserialize, Debug)]
 pub struct RuntimeMetadata {
     /// The cargo package version from this library's cargo.toml, e.g., "1.2.3".
     pub rayhunter_version: String,