Fix: Voice claim system - radio stops when soundboard plays

Plugins now claim voice per guild via claimVoice(). When soundboard
plays a sound, radio's cleanup runs automatically (kills ffmpeg,
broadcasts SSE stop event). Fixes stale "now playing" UI on tab switch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/core/plugin.ts

@@ -39,3 +39,25 @@ export function registerPlugin(plugin: Plugin): void {
 export function getPlugins(): Plugin[] {
   return [...loadedPlugins];
 }
+
+// ── Voice claim system ──
+// Only one plugin can use voice per guild. When a new plugin claims voice,
+// the previous claimant's cleanup callback is invoked automatically.
+type VoiceClaimCleanup = () => void;
+const voiceClaims = new Map<string, { plugin: string; cleanup: VoiceClaimCleanup }>();
+
+export function claimVoice(guildId: string, pluginName: string, cleanup: VoiceClaimCleanup): void {
+  const existing = voiceClaims.get(guildId);
+  if (existing && existing.plugin !== pluginName) {
+    console.log(`[Voice] ${pluginName} claims guild ${guildId}, releasing ${existing.plugin}`);
+    existing.cleanup();
+  }
+  voiceClaims.set(guildId, { plugin: pluginName, cleanup });
+}
+
+export function releaseVoice(guildId: string, pluginName: string): void {
+  const claim = voiceClaims.get(guildId);
+  if (claim?.plugin === pluginName) {
+    voiceClaims.delete(guildId);
+  }
+}