refactor(media): remove volume control from FE & BE

Volume sudah di-set default 0.3 di gateway (suara kecil saat play), dan
user bisa naikin sendiri di Discord (command media:volume) — jadi kontrol
volume lewat dashboard tak perlu. Hapus:
- BE: POST /api/media/volume route, mediaVolumeSchema, setVolume service
- FE: useMediaVolume hook, mediaApi.volume, slider volume di music-player
  + mini-player, field volume/setVolume di MediaPlayerProvider
Pertahankan COMMAND_MEDIA_VOLUME di gateway (masih dipakai command DC)
dan mic volume (terpisah, tetap di voice page).

services/backend/src/modules/media/media.routes.ts

@@ -2,8 +2,8 @@ import type { Request, Response, Router } from "express";
 import express from "express";
 import { createChildLogger } from "@/shared/logger/index";
 import { asyncHandler, validateBody } from "../../shared/middlewares/index.js";
-import { mediaQueueSchema, mediaVolumeSchema } from "./media.schema.js";
-import { getStatus, queue, setVolume, skip, stop } from "./media.service.js";
+import { mediaQueueSchema } from "./media.schema.js";
+import { getStatus, queue, skip, stop } from "./media.service.js";
 
 const logger = createChildLogger("media.routes");
 
@@ -55,17 +55,5 @@ export function createMediaRouter(): Router {
     }),
   );
 
-  // POST /api/media/volume
-  router.post(
-    "/media/volume",
-    validateBody(mediaVolumeSchema),
-    asyncHandler(async (req: Request, res: Response) => {
-      const { volume } = req.body as { volume: number };
-      logger.debug({ volume }, "Media volume requested");
-      const state = await setVolume(volume);
-      res.json(state);
-    }),
-  );
-
   return router;
 }