refactor: remove mut requirement from get_scrobble function

RwLock uses interior mutability magic to ensure that you don't need a
&mut reference for write() to work.

server/src/index.rs

···
     scrobble: Option<ScrobblesTemplate>,
 }
 
-pub async fn get_index(mut monitor: ScrobbleMonitor) -> RootTemplate {
+pub async fn get_index(monitor: ScrobbleMonitor) -> RootTemplate {
     let scrobbles_template = monitor.try_get_scrobble();
     if scrobbles_template.is_none() {
         // start fetching scrobble so we can send a fresh response to the client ASAP

server/src/main.rs

···
 }
 
 async fn get_scrobble(
-    State(mut monitor): State<ScrobbleMonitor>,
+    State(monitor): State<ScrobbleMonitor>,
     Query(ScrobbleQuery { immediate }): Query<ScrobbleQuery>,
 ) -> Sse<impl Stream<Item = Result<sse::Event, Infallible>>> {
     let stream = stream! {

server/src/scrobble_monitor.rs

···
             .map(|scrobble| scrobble.data.clone())
     }
 
-    pub async fn get_scrobble(&mut self) -> anyhow::Result<ScrobblesTemplate> {
+    pub async fn get_scrobble(&self) -> anyhow::Result<ScrobblesTemplate> {
         if let Some(scrobble) = &*self.last_scrobble.read().await {
             if scrobble.is_fresh() {
                 tracing::debug!("returning recently fetched scrobble data");