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/api/server.ts

@@ -186,6 +186,7 @@ export async function startApiServer({ manager, context }: ApiServerOptions) {
       features: {
         localLibrary: Boolean(config.LOCAL_MEDIA_DIR),
         requireListener: config.REQUIRE_LISTENER,
+        video: config.VIDEO_ENABLED,
       },
     });
   });
@@ -265,6 +266,8 @@ export async function startApiServer({ manager, context }: ApiServerOptions) {
     leave: (serverId, userId) => manager.leave(serverId, userId),
     volume: (serverId, userId, body) =>
       manager.setVolume(serverId, userId, z.object({ volume: z.number().min(0).max(200) }).parse(body).volume),
+    video: (serverId, userId, body) =>
+      manager.setVideo(serverId, userId, z.object({ enabled: z.boolean() }).parse(body).enabled),
     loop: (serverId, userId, body) =>
       manager.setLoop(serverId, userId, z.object({ mode: z.enum(["off", "track", "queue"]) }).parse(body).mode),
     seek: (serverId, userId, body) =>