feat(api): add cursor-based pagination for posts, comments, messages, groups, and notifications

Implement cursor-based pagination across multiple API endpoints to improve performance and enable efficient infinite scrolling. This replaces traditional offset-based pagination for high-volume data retrieval.

Changes:
- Add cursor pagination DTOs for all entity types (Post, Comment, Message, Conversation, Group, Notification, GroupMember, GroupAnnouncement)
- Implement cursor pagination methods in repositories with support for various sort orders (created_at, updated_at, join_time, seq)
- Add cursor pagination handlers that maintain backward compatibility with existing offset pagination
- Add new API routes for cursor-based endpoints (/cursor suffix)
- Add helper converter functions for pointer slice types in DTO conversions

internal/repository/notification_repo.go

@@ -2,6 +2,8 @@ package repository
 
 import (
 	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/cursor"
+	"context"
 
 	"gorm.io/gorm"
 )
@@ -76,3 +78,63 @@ func (r *NotificationRepository) GetUnreadCount(userID string) (int64, error) {
 func (r *NotificationRepository) DeleteAllByUserID(userID string) error {
 	return r.db.Where("user_id = ?", userID).Delete(&model.Notification{}).Error
 }
+
+// ========== Cursor Pagination Methods ==========
+
+// GetNotificationsByCursor 游标分页获取用户通知
+// 排序方式：created_at DESC
+func (r *NotificationRepository) GetNotificationsByCursor(ctx context.Context, userID string, unreadOnly bool, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Notification], error) {
+	// 构建基础查询
+	query := r.db.WithContext(ctx).Model(&model.Notification{}).Where("user_id = ?", userID)
+
+	if unreadOnly {
+		query = query.Where("is_read = ?", false)
+	}
+
+	// 使用游标构建器
+	builder := cursor.NewBuilder(query, cursor.SortByCreatedAtDesc).
+		WithCursor(req.Cursor, req.Direction).
+		WithPageSize(req.PageSize)
+
+	if builder.Error() != nil {
+		// 无效游标，返回空列表
+		return cursor.NewCursorPageResult[*model.Notification]([]*model.Notification{}, "", "", false), nil
+	}
+
+	// 执行查询
+	var notifications []*model.Notification
+	query = builder.Build()
+	if err := query.Find(&notifications).Error; err != nil {
+		return nil, err
+	}
+
+	// 构建响应
+	pageSize := builder.GetPageSize()
+	hasMore := cursor.HasMore(len(notifications), pageSize)
+	if hasMore {
+		notifications = notifications[:pageSize]
+	}
+
+	// 生成游标
+	var nextCursor, prevCursor string
+	if len(notifications) > 0 {
+		// 下一页游标
+		if hasMore {
+			lastNotification := notifications[len(notifications)-1]
+			nextCursor = cursor.NewCursor(
+				cursor.FormatTime(lastNotification.CreatedAt),
+				lastNotification.ID,
+				cursor.SortByCreatedAtDesc,
+			).Encode()
+		}
+		// 上一页游标
+		firstNotification := notifications[0]
+		prevCursor = cursor.NewCursor(
+			cursor.FormatTime(firstNotification.CreatedAt),
+			firstNotification.ID,
+			cursor.SortByCreatedAtDesc,
+		).Encode()
+	}
+
+	return cursor.NewCursorPageResult(notifications, nextCursor, prevCursor, hasMore), nil
+}