Make video a per-server toggle instead of an env-wide setting

VIDEO_ENABLED is now permission rather than behaviour: it decides whether
the feature exists at all, while turning it on for a server is a toggle in
the panel or !video in chat, off by default. Video costs real CPU for
every playing channel, so that should be a deliberate choice rather than
something a config flag switches on everywhere.

With the env flag off the panel renders no toggle at all and !video says
so, and the switch applies from the next track — swapping tracks mid-play
would cut the current one.

The loop button no longer reads "выкл" either: it sat next to the video
button showing the same word, so the two states were indistinguishable.
Both now name what they do and rely on highlighting for state.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/manager.ts

@@ -269,6 +269,10 @@ export class MusicManager extends EventEmitter<ManagerEvents> {
     (await this.require(serverId, userId)).setVolume(volume);
   }
 
+  async setVideo(serverId: string, userId: string, enabled: boolean): Promise<void> {
+    (await this.require(serverId, userId)).setVideo(enabled);
+  }
+
   async setLoop(serverId: string, userId: string, mode: LoopMode): Promise<void> {
     (await this.require(serverId, userId)).setLoop(mode);
   }
@@ -314,6 +318,7 @@ export class MusicManager extends EventEmitter<ManagerEvents> {
       volume: config.DEFAULT_VOLUME,
       loop: "off",
       shuffleUsed: false,
+      videoEnabled: false,
       updatedAt: Date.now(),
     };
   }