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/sources/index.ts

@@ -173,9 +173,14 @@ export interface PlaybackInput {
   video?: { stream: Readable; width: number; height: number };
 }
 
+export interface PlaybackOptions {
+  /** Publish the picture too; the server-wide switch still has to allow it. */
+  video?: boolean;
+}
+
 /** Only YouTube reliably carries a picture worth showing next to the audio. */
-function canShowVideo(track: Track): boolean {
-  return config.VIDEO_ENABLED && track.source === "youtube" && !track.isLive;
+function canShowVideo(track: Track, wanted: boolean): boolean {
+  return wanted && config.VIDEO_ENABLED && track.source === "youtube" && !track.isLive;
 }
 
 const HTTP_RESILIENCE = [
@@ -196,7 +201,11 @@ function ffmpegProxyOptions(): string[] {
 }
 
 /** Opens an ffmpeg-compatible input for a track, optionally starting at an offset. */
-export async function openPlayback(track: Track, seekSeconds = 0): Promise<PlaybackInput> {
+export async function openPlayback(
+  track: Track,
+  seekSeconds = 0,
+  options: PlaybackOptions = {},
+): Promise<PlaybackInput> {
   const seekOptions = seekSeconds > 0 ? ["-ss", seekSeconds.toFixed(2)] : [];
 
   if (track.source === "local") {
@@ -214,7 +223,10 @@ export async function openPlayback(track: Track, seekSeconds = 0): Promise<Playb
 
   // The format is checked before committing to the video pipeline: audio comes
   // out of that same pipeline, so falling back afterwards would kill the sound.
-  if (canShowVideo(track) && (await ytdlp.hasProgressiveVideo(track.url, config.VIDEO_HEIGHT))) {
+  if (
+    canShowVideo(track, options.video ?? false) &&
+    (await ytdlp.hasProgressiveVideo(track.url, config.VIDEO_HEIGHT))
+  ) {
     log.info({ title: track.title }, "playing with video");
     const pipeline = openVideoPipeline(track.url, seekSeconds);
     return {