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

backend/handler/leaderboard.go

@@ -0,0 +1,71 @@
+package handler
+
+import (
+	"net/http"
+	"strconv"
+
+	"github.com/freefire/jiang13-bbs/service"
+	"github.com/gin-gonic/gin"
+)
+
+// 榜单参数白名单
+var (
+	lbPeriods = map[string]bool{service.PeriodWeek: true, service.PeriodMonth: true, service.PeriodAll: true}
+	lbMetrics = map[string]bool{service.MetricAll: true, service.MetricPost: true,
+		service.MetricComment: true, service.MetricLike: true, service.MetricAnswer: true}
+)
+
+// lbParams 解析并校验 period/metric，非法值回退默认并标记 400
+func lbParams(c *gin.Context) (period, metric string, bad bool) {
+	period = c.DefaultQuery("period", service.PeriodAll)
+	metric = c.DefaultQuery("metric", service.MetricAll)
+	if !lbPeriods[period] || !lbMetrics[metric] {
+		return "", "", true
+	}
+	return period, metric, false
+}
+
+// Leaderboard 用户贡献榜：period=week|month|all × metric=all|post|comment|like|answer（公开接口）
+func (h *Handlers) Leaderboard(c *gin.Context) {
+	period, metric, bad := lbParams(c)
+	if bad {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的榜单参数"})
+		return
+	}
+	limit := 50
+	if v := c.Query("limit"); v != "" {
+		if n, err := strconv.Atoi(v); err == nil && n > 0 {
+			limit = n
+		}
+	}
+	if limit > 100 {
+		limit = 100
+	}
+	res, err := h.LeaderboardSvc.Board(period, metric, limit)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "获取排行榜失败"})
+		return
+	}
+	c.JSON(http.StatusOK, res)
+}
+
+// LeaderboardMe 当前用户在指定榜单的排名（可选登录；游客返回 me:null）
+func (h *Handlers) LeaderboardMe(c *gin.Context) {
+	period, metric, bad := lbParams(c)
+	if bad {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的榜单参数"})
+		return
+	}
+	if v, ok := c.Get("user"); ok {
+		if claims, ok := v.(*service.UserClaims); ok && claims.ID > 0 {
+			res, err := h.LeaderboardSvc.MyRank(period, metric, claims.ID)
+			if err != nil {
+				c.JSON(http.StatusInternalServerError, gin.H{"error": "获取排名失败"})
+				return
+			}
+			c.JSON(http.StatusOK, gin.H{"me": res})
+			return
+		}
+	}
+	c.JSON(http.StatusOK, gin.H{"me": nil})
+}