feat: 新增视频播放页面

- 新增 /video/:id 路由与 VideoPage 组件
- 视频文件通过 /uploads/videos/{id}.mp4 静态访问
- .gitignore 排除 videos 目录，避免大文件入库

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

packages/web/src/pages/VideoPage.tsx

@@ -0,0 +1,22 @@
+import { useParams } from "react-router-dom";
+
+export default function VideoPage() {
+  const { id } = useParams();
+  const src = id ? `/uploads/videos/${id}.mp4` : "";
+
+  return (
+    <div className="min-h-svh flex items-center justify-center bg-black px-4 py-4">
+      {id ? (
+        <video
+          src={src}
+          controls
+          playsInline
+          preload="metadata"
+          className="block w-full max-w-[960px] h-auto bg-black"
+        />
+      ) : (
+        <p className="text-white/60 text-sm">视频不存在</p>
+      )}
+    </div>
+  );
+}