feat(admin): add comprehensive admin management system

- Add admin handlers and services for users, posts, comments, groups, and dashboard
- Implement role permission management with ability to view and update permissions
- Add database seeding for roles and Casbin permission policies
- Upgrade Casbin from v2 to v3 with GORM adapter for persistent policy storage
- Add admin-specific repository methods with filtering and pagination
- Register new admin API routes under /api/v1/admin/*

internal/handler/admin_dashboard_handler.go

@@ -0,0 +1,100 @@
+package handler
+
+import (
+	"strconv"
+
+	"carrot_bbs/internal/pkg/response"
+	"carrot_bbs/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+// AdminDashboardHandler 管理端仪表盘处理器
+type AdminDashboardHandler struct {
+	dashboardService service.AdminDashboardService
+}
+
+// NewAdminDashboardHandler 创建管理端仪表盘处理器
+func NewAdminDashboardHandler(dashboardService service.AdminDashboardService) *AdminDashboardHandler {
+	return &AdminDashboardHandler{
+		dashboardService: dashboardService,
+	}
+}
+
+// GetStats 获取统计数据
+// GET /api/v1/admin/dashboard/stats
+func (h *AdminDashboardHandler) GetStats(c *gin.Context) {
+	stats, err := h.dashboardService.GetStats(c.Request.Context())
+	if err != nil {
+		response.InternalServerError(c, "failed to get dashboard stats")
+		return
+	}
+
+	response.Success(c, stats)
+}
+
+// GetUserActivityTrend 获取用户活跃度趋势
+// GET /api/v1/admin/dashboard/user-activity
+func (h *AdminDashboardHandler) GetUserActivityTrend(c *gin.Context) {
+	// 解析天数参数，默认7天
+	days, _ := strconv.Atoi(c.DefaultQuery("days", "7"))
+	if days <= 0 {
+		days = 7
+	}
+	if days > 30 {
+		days = 30
+	}
+
+	trend, err := h.dashboardService.GetUserActivityTrend(c.Request.Context(), days)
+	if err != nil {
+		response.InternalServerError(c, "failed to get user activity trend")
+		return
+	}
+
+	response.Success(c, trend)
+}
+
+// GetContentStats 获取内容统计
+// GET /api/v1/admin/dashboard/content-stats
+func (h *AdminDashboardHandler) GetContentStats(c *gin.Context) {
+	stats, err := h.dashboardService.GetContentStats(c.Request.Context())
+	if err != nil {
+		response.InternalServerError(c, "failed to get content stats")
+		return
+	}
+
+	response.Success(c, stats)
+}
+
+// GetPendingContent 获取待审核内容
+// GET /api/v1/admin/dashboard/pending-content
+func (h *AdminDashboardHandler) GetPendingContent(c *gin.Context) {
+	// 解析limit参数，默认10条
+	limit, _ := strconv.Atoi(c.DefaultQuery("limit", "10"))
+	if limit <= 0 {
+		limit = 10
+	}
+	if limit > 50 {
+		limit = 50
+	}
+
+	content, err := h.dashboardService.GetPendingContent(c.Request.Context(), limit)
+	if err != nil {
+		response.InternalServerError(c, "failed to get pending content")
+		return
+	}
+
+	response.Success(c, content)
+}
+
+// GetOnlineUsers 获取在线用户数
+// GET /api/v1/admin/dashboard/online-users
+func (h *AdminDashboardHandler) GetOnlineUsers(c *gin.Context) {
+	onlineUsers, err := h.dashboardService.GetOnlineUsers(c.Request.Context())
+	if err != nil {
+		response.InternalServerError(c, "failed to get online users count")
+		return
+	}
+
+	response.Success(c, onlineUsers)
+}