main: Implement exponential backoff in the feed fetcher loop

Backoff with an exponentially increasing delay when feeds can't be
fetched or parsed, with a maximum of 6h (after which retry every 6h)

src/matrix.rs

@@ -98,7 +98,7 @@ pub async fn login_and_sync<T: Clone>(
     client
         .matrix_auth()
         .login_username(username, password)
-        .initial_device_display_name("bender v0.1.3")
+        .initial_device_display_name("bender v0.1.4")
         .await?;
 
     info!("logged in as {username}");