feat(readarr): add the Readarr health check network call

Inhabit ReadarrEvent with its first variant, HealthCheck, mapped to
GET /health, and add get_readarr_healthcheck alongside the equivalent
Lidarr, Radarr, and Sonarr fetchers. The call is internal only and has
no CLI surface, matching the other three servarrs; it exists so that an
unreachable instance surfaces an error banner on tick.

The healthcheck fetcher lives in readarr_network/mod.rs rather than a
submodule, so its tests live in readarr_network_tests.rs next to the
resource() and NetworkEvent conversion tests, mirroring Lidarr.

MockServarrApi::build_for matched NetworkEvent by value with no Radarr
fallback arm. It compiled only while ReadarrEvent was uninhabited and
no value could reach it; the first variant makes the match non
exhaustive, so it now assigns the config to the Readarr server tab.

src/network/network_tests.rs

@@ -992,6 +992,7 @@ pub(in crate::network) mod test_utils {
         NetworkEvent::Radarr(_) => app.server_tabs.tabs[0].config = Some(servarr_config),
         NetworkEvent::Sonarr(_) => app.server_tabs.tabs[1].config = Some(servarr_config),
         NetworkEvent::Lidarr(_) => app.server_tabs.tabs[2].config = Some(servarr_config),
+        NetworkEvent::Readarr(_) => app.server_tabs.tabs[3].config = Some(servarr_config),
       }
 
       let app_arc = Arc::new(Mutex::new(app));