Add GET /music/{guildId}/volume endpoint

Introduced a new endpoint to retrieve the current volume level for a guild's music player. Updated `MusicRoutes` and `MusicManager` with the necessary logic and added OpenAPI documentation for the route. This enhances API completeness by providing both read and write access to the volume setting.

src/main/java/me/bobert/netneutrality/http/server/MusicRoutes.java

@@ -72,6 +72,7 @@ public final class MusicRoutes {
         routes.post("/music/{guildId}/stop", ctx -> stop(jdaSupplier, musicService, ctx));
         routes.post("/music/{guildId}/pause", ctx -> pause(jdaSupplier, musicService, ctx));
         routes.post("/music/{guildId}/resume", ctx -> resumePlayback(jdaSupplier, musicService, ctx));
+        routes.get("/music/{guildId}/volume", ctx -> getVolume(jdaSupplier, musicService, ctx));
         routes.put("/music/{guildId}/volume", ctx -> volume(jdaSupplier, musicService, ctx));
         routes.put("/music/{guildId}/seek", ctx -> seek(jdaSupplier, musicService, ctx));
         routes.post("/music/{guildId}/vote-skip/{userId}", ctx -> voteSkip(jdaSupplier, musicService, ctx));
@@ -218,6 +219,22 @@ public final class MusicRoutes {
         ctx.json(Map.of("status", "ok", "message", "Resuming current track"));
     }
 
+    @OpenApi(
+        path = "/music/{guildId}/volume",
+        methods = HttpMethod.GET,
+        summary = "Get the current player volume of a guild",
+        tags = {"Music"},
+        pathParams = @OpenApiParam(name = "guildId", description = "The Discord guild id"),
+        responses = {
+            @OpenApiResponse(status = "200", content = @OpenApiContent(from = VolumeResponse.class)),
+            @OpenApiResponse(status = "404", content = @OpenApiContent(from = ErrorResponse.class))
+        }
+    )
+    private static void getVolume(Supplier<JDA> jdaSupplier, MusicService musicService, Context ctx) {
+        MusicManager manager = manager(jdaSupplier, musicService, ctx);
+        ctx.json(Map.of("status", "ok", "volume", manager.getVolume()));
+    }
+
     @OpenApi(
         path = "/music/{guildId}/volume",
         methods = HttpMethod.PUT,

src/main/java/me/bobert/netneutrality/music/MusicManager.java

@@ -367,6 +367,18 @@ public class MusicManager implements MusicManagerCallback {
         updatePlayer("set volume", builder -> builder.setVolume(volume));
     }
 
+    /**
+     * Returns the player's current volume on the Lavalink node.
+     *
+     * @return the volume, from {@code 0} to {@code 1000}, or {@link #DEFAULT_VOLUME} when no
+     *         player is cached
+     */
+    public int getVolume() {
+        Link link = getLink();
+        LavalinkPlayer player = link == null ? null : link.getCachedPlayer();
+        return player == null ? DEFAULT_VOLUME : player.getVolume();
+    }
+
     /**
      * Seeks the currently playing track to the given position.
      *