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]/events/route.ts

@@ -0,0 +1,76 @@
+import { buildRoomStatus } from "@/lib/buildRoomStatus";
+import { subscribe } from "@/lib/roomEvents";
+
+export const dynamic = "force-dynamic";
+
+export async function GET(
+  req: Request,
+  { params }: { params: Promise<{ id: string }> },
+) {
+  const { id } = await params;
+
+  const encoder = new TextEncoder();
+
+  const stream = new ReadableStream({
+    start(controller) {
+      const send = (obj: object) => {
+        try {
+          controller.enqueue(
+            encoder.encode(`data: ${JSON.stringify(obj)}\n\n`),
+          );
+        } catch {
+          /* controller already closed */
+        }
+      };
+
+      let alive = true;
+
+      (async () => {
+        const status = await buildRoomStatus(id);
+        if (!status) {
+          send({ error: "room_not_found" });
+          controller.close();
+          return;
+        }
+        if (alive) send(status);
+      })();
+
+      const unsubscribe = subscribe(id, async () => {
+        if (!alive) return;
+        try {
+          const status = await buildRoomStatus(id);
+          if (status && alive) send(status);
+        } catch {
+          /* ignore transient read errors */
+        }
+      });
+
+      const heartbeat = setInterval(() => {
+        try {
+          controller.enqueue(encoder.encode(": heartbeat\n\n"));
+        } catch {
+          clearInterval(heartbeat);
+        }
+      }, 30_000);
+
+      req.signal.addEventListener("abort", () => {
+        alive = false;
+        unsubscribe();
+        clearInterval(heartbeat);
+        try {
+          controller.close();
+        } catch {
+          /* already closed */
+        }
+      });
+    },
+  });
+
+  return new Response(stream, {
+    headers: {
+      "Content-Type": "text/event-stream",
+      "Cache-Control": "no-cache, no-transform",
+      Connection: "keep-alive",
+    },
+  });
+}