feat: 用 SSE 替代 SWR 轮询，实现房间状态实时推送

SSE 断连时自动降级为 2s 轮询，重连后切回 SSE。

src/hooks/useRoomPolling.ts

@@ -1,28 +1,55 @@
 "use client";
 
 import useSWR from "swr";
-import { useRef } from "react";
+import { useEffect, useRef } from "react";
 import { RoomStatus } from "@/types";
 
 const fetcher = (url: string) => fetch(url).then((r) => r.json());
 
 export function useRoomPolling(roomId: string) {
-  const settled = useRef(false);
-
   const { data, error, isLoading, mutate } = useSWR<RoomStatus>(
     `/api/room/${roomId}`,
     fetcher,
-    {
-      refreshInterval: settled.current ? 0 : 1500,
-      revalidateOnFocus: true,
-    },
+    { revalidateOnFocus: true },
   );
 
-  if (data?.match != null) {
-    settled.current = true;
-  } else {
-    settled.current = false;
-  }
+  const fallbackRef = useRef<ReturnType<typeof setInterval> | null>(null);
+
+  useEffect(() => {
+    const es = new EventSource(`/api/room/${roomId}/events`);
+
+    es.onmessage = (e) => {
+      try {
+        const parsed = JSON.parse(e.data);
+        if (parsed.roomId) {
+          mutate(parsed, { revalidate: false });
+        }
+      } catch {
+        /* malformed message */
+      }
+    };
+
+    es.onerror = () => {
+      if (!fallbackRef.current) {
+        fallbackRef.current = setInterval(() => mutate(), 2000);
+      }
+    };
+
+    es.onopen = () => {
+      if (fallbackRef.current) {
+        clearInterval(fallbackRef.current);
+        fallbackRef.current = null;
+      }
+    };
+
+    return () => {
+      es.close();
+      if (fallbackRef.current) {
+        clearInterval(fallbackRef.current);
+        fallbackRef.current = null;
+      }
+    };
+  }, [roomId, mutate]);
 
   return {
     userCount: data?.userCount ?? 0,