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/readarr_network/mod.rs

@@ -1,15 +1,24 @@
 use anyhow::Result;
+use log::info;
 
 use super::{NetworkEvent, NetworkResource};
 use crate::models::readarr_models::ReadarrSerdeable;
-use crate::network::Network;
+use crate::network::{Network, RequestMethod};
+
+#[cfg(test)]
+#[path = "readarr_network_tests.rs"]
+mod readarr_network_tests;
 
 #[derive(Debug, Eq, PartialEq, Clone)]
-pub enum ReadarrEvent {}
+pub enum ReadarrEvent {
+  HealthCheck,
+}
 
 impl NetworkResource for ReadarrEvent {
   fn resource(&self) -> &'static str {
-    match *self {}
+    match &self {
+      ReadarrEvent::HealthCheck => "/health",
+    }
   }
 }
 
@@ -24,6 +33,26 @@ impl Network<'_, '_> {
     &mut self,
     readarr_event: ReadarrEvent,
   ) -> Result<ReadarrSerdeable> {
-    match readarr_event {}
+    match readarr_event {
+      ReadarrEvent::HealthCheck => self
+        .get_readarr_healthcheck()
+        .await
+        .map(ReadarrSerdeable::from),
+    }
+  }
+
+  pub(in crate::network::readarr_network) async fn get_readarr_healthcheck(
+    &mut self,
+  ) -> Result<()> {
+    info!("Performing Readarr health check");
+    let event = ReadarrEvent::HealthCheck;
+
+    let request_props = self
+      .request_props_from(event, RequestMethod::Get, None::<()>, None, None)
+      .await;
+
+    self
+      .handle_request::<(), ()>(request_props, |_, _| ())
+      .await
   }
 }