feat: SSE live fight spectating with spectator count

Enable real-time fight spectating for all live fights (not just human
fights). Multiple spectators can watch simultaneously via SSE. Spectator
count is tracked per-fight and broadcast with every SSE event.

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

frontend/src/components/SpritePreview.vue

@@ -55,6 +55,8 @@ watch(() => [props.seed, props.archetype, props.customization, props.pose], () =
 
 onUnmounted(() => {
   if (animHandle) clearTimeout(animHandle)
+  animHandle = null
+  if (img) { img.onload = null; img.src = ''; img = null }
 })
 </script>