feat: 新增排行榜模块

- 新增 leaderboard 服务与 handler、前端排行榜页面
- 路由与首页入口适配

frontend/lib/api.ts

@@ -759,6 +759,79 @@ export async function fetchBoardSidebar(
   return res.json();
 }
 
+/** 榜单周期：本周 | 本月 | 总榜 */
+export type LeaderboardPeriod = "week" | "month" | "all";
+/** 榜单维度：全部（综合）| 发帖最多 | 回复最多 | 获赞最多 | 最佳答案 */
+export type LeaderboardMetric = "all" | "post" | "comment" | "like" | "answer";
+
+/** 排行榜单条（领奖台/列表通用；level 由后端按累计积分计算） */
+export interface LeaderboardEntry {
+  rank: number;
+  id: number;
+  username: string;
+  nickname: string;
+  avatar: string;
+  total_points: number;
+  points: number;
+  level: number;
+  post_count: number;
+  comment_count: number;
+  like_count: number;
+  answer_count: number;
+  score: number;
+}
+
+/** /api/leaderboard 响应 */
+export interface LeaderboardResponse {
+  period: LeaderboardPeriod;
+  metric: LeaderboardMetric;
+  entries: LeaderboardEntry[];
+}
+
+/** 我的排名（rank=0 未上榜；change 正=上升负=下降，has_change=false 表示无上一周期对比） */
+export interface LeaderboardMe {
+  period: LeaderboardPeriod;
+  metric: LeaderboardMetric;
+  rank: number;
+  total: number;
+  score: number;
+  change: number;
+  has_change: boolean;
+}
+
+/** SSR 排行榜；失败时抛错，由页面降级为空榜 */
+export async function fetchLeaderboard(
+  period: LeaderboardPeriod,
+  metric: LeaderboardMetric,
+  cookieHeader?: string
+): Promise<LeaderboardResponse> {
+  const res = await fetch(
+    `${API_BASE}/api/leaderboard?period=${period}&metric=${metric}`,
+    ssrInit(cookieHeaders(cookieHeader))
+  );
+  if (!res.ok) throw new Error("获取排行榜失败");
+  return res.json();
+}
+
+/** SSR 我的排名；游客/失败返回 null（不抛错） */
+export async function fetchMyRank(
+  period: LeaderboardPeriod,
+  metric: LeaderboardMetric,
+  cookieHeader?: string
+): Promise<LeaderboardMe | null> {
+  try {
+    const res = await fetch(
+      `${API_BASE}/api/leaderboard/me?period=${period}&metric=${metric}`,
+      ssrInit(cookieHeaders(cookieHeader))
+    );
+    if (!res.ok) return null;
+    const data = (await res.json()) as { me: LeaderboardMe | null };
+    return data.me ?? null;
+  } catch {
+    return null;
+  }
+}
+
 export async function fetchBoards(): Promise<{ boards: BoardWithCount[] }> {
   const res = await fetch(`${API_BASE}/api/boards`, ssrInit());
   if (!res.ok) throw new Error("获取板块失败");