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/service/notification_service.go

@@ -7,6 +7,7 @@ import (
 	"carrot_bbs/internal/cache"
 	apperrors "carrot_bbs/internal/errors"
 	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/cursor"
 	"carrot_bbs/internal/repository"
 )
 
@@ -166,5 +167,10 @@ func (s *NotificationService) ClearAllNotifications(ctx context.Context, userID
 	return nil
 }
 
+// GetNotificationsByCursor 游标分页获取用户通知
+func (s *NotificationService) GetNotificationsByCursor(ctx context.Context, userID string, unreadOnly bool, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Notification], error) {
+	return s.notificationRepo.GetNotificationsByCursor(ctx, userID, unreadOnly, req)
+}
+
 // 错误定义
 var ErrUnauthorizedNotification = apperrors.ErrUnauthorizedNotification