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/app/api/room/[id]/reset/route.ts

@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { atomicUpdateRoom } from "@/lib/store";
+import { notify } from "@/lib/roomEvents";
 
 export async function POST(
   req: Request,
@@ -36,6 +37,8 @@ export async function POST(
       );
     }
 
+    notify(id);
+
     return NextResponse.json({ ok: true });
   } catch (e) {
     console.error("Failed to reset room:", e);