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/repository/role_repo.go

@@ -30,6 +30,9 @@ type RoleRepository interface {
 
 	// GetAllRoles 获取所有角色
 	GetAllRoles(ctx context.Context) ([]model.Role, error)
+
+	// GetRoleUserCount 获取角色对应的用户数量
+	GetRoleUserCount(ctx context.Context, role string) (int64, error)
 }
 
 type roleRepository struct {
@@ -95,3 +98,12 @@ func (r *roleRepository) GetAllRoles(ctx context.Context) ([]model.Role, error)
 	err := r.db.WithContext(ctx).Order("priority DESC").Find(&roles).Error
 	return roles, err
 }
+
+func (r *roleRepository) GetRoleUserCount(ctx context.Context, role string) (int64, error) {
+	var count int64
+	err := r.db.WithContext(ctx).
+		Model(&model.UserRole{}).
+		Where("role = ?", role).
+		Count(&count).Error
+	return count, err
+}