chore: 清理 SSR 改造后的历史遗留死代码与误提交产物

- 删除图形验证码整条死链：service/captcha.go、APICaptcha、
  GET /api/captcha、CaptchaService 装配、ErrCaptchaInvalid 及前端
  api.captcha()（注册/登录已改用邮箱验证码，Verify 无任何调用方）
- 删除 GET /api/posts/hot 死路由：APIHotPosts + PostService.HotPosts
  （热门排序走 /api/posts?sort=hot）
- 删除 GET /api/posts/:id/revisions/:revId 死路由：
  APIPostRevisionDetail + PostService.GetRevision + ErrRevisionNotFound
  及前端 api.postRevision()（历史面板仅用列表接口）
- 停止跟踪误提交的 27.5MB Linux 二进制 jiang13-linux-amd64 与
  .vite 依赖缓存；.gitignore 补充对应规则并移除已失效的
  embed_static 条目

frontend/src/api/client.ts

@@ -264,8 +264,6 @@ export const api = {
     }),
   postRevisions: (id: number) =>
     request<{ revisions: PostRevision[] }>(`/api/posts/${id}/revisions`),
-  postRevision: (id: number, revId: number) =>
-    request<{ revision: PostRevision }>(`/api/posts/${id}/revisions/${revId}`),
   adminDeletePost: (id: number) => request(`/api/admin/posts/${id}`, { method: 'DELETE' }),
   adminTrashPosts: (params: { page?: number; keyword?: string }) => {
     const q = new URLSearchParams();
@@ -582,7 +580,6 @@ export const api = {
       `/api/users/search?${sp}`,
     );
   },
-  captcha: () => request<{ id: string; image: string }>('/api/captcha'),
   logout: () => request('/api/logout', { method: 'POST' }),
   like: (id: number) => request<{ liked: boolean; like_count: number }>(`/api/posts/${id}/like`, { method: 'POST' }),
   likeComment: (id: number) => request<{ liked: boolean; like_count: number }>(`/api/comments/${id}/like`, { method: 'POST' }),