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/group_service.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"cmp"
+	"context"
 	"errors"
 	"fmt"
 	"maps"
@@ -12,6 +13,7 @@ import (
 	"carrot_bbs/internal/cache"
 	apperrors "carrot_bbs/internal/errors"
 	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/cursor"
 	"carrot_bbs/internal/pkg/sse"
 	"carrot_bbs/internal/pkg/utils"
 	"carrot_bbs/internal/repository"
@@ -88,6 +90,12 @@ type GroupService interface {
 
 	// 获取成员信息
 	GetMember(groupID string, userID string) (*model.GroupMember, error)
+
+	// 游标分页方法
+	GetGroupsByCursor(ctx context.Context, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Group], error)
+	GetUserGroupsByCursor(ctx context.Context, userID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Group], error)
+	GetMembersByCursor(ctx context.Context, groupID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.GroupMember], error)
+	GetAnnouncementsByCursor(ctx context.Context, groupID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.GroupAnnouncement], error)
 }
 
 // GroupMembersResult 群组成员缓存结果
@@ -1614,3 +1622,25 @@ func (s *groupService) IsGroupOwner(userID string, groupID string) bool {
 func (s *groupService) GetMember(groupID string, userID string) (*model.GroupMember, error) {
 	return s.groupRepo.GetMember(groupID, userID)
 }
+
+// ==================== 游标分页方法 ====================
+
+// GetGroupsByCursor 游标分页获取群组列表
+func (s *groupService) GetGroupsByCursor(ctx context.Context, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Group], error) {
+	return s.groupRepo.GetGroupsByCursor(ctx, req)
+}
+
+// GetUserGroupsByCursor 游标分页获取用户加入的群组列表
+func (s *groupService) GetUserGroupsByCursor(ctx context.Context, userID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Group], error) {
+	return s.groupRepo.GetUserGroupsByCursor(ctx, userID, req)
+}
+
+// GetMembersByCursor 游标分页获取群成员列表
+func (s *groupService) GetMembersByCursor(ctx context.Context, groupID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.GroupMember], error) {
+	return s.groupRepo.GetMembersByCursor(ctx, groupID, req)
+}
+
+// GetAnnouncementsByCursor 游标分页获取群公告列表
+func (s *groupService) GetAnnouncementsByCursor(ctx context.Context, groupID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.GroupAnnouncement], error) {
+	return s.groupRepo.GetAnnouncementsByCursor(ctx, groupID, req)
+}