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/message_service.go

@@ -6,6 +6,7 @@ import (
 
 	"carrot_bbs/internal/cache"
 	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/cursor"
 	"carrot_bbs/internal/repository"
 
 	"go.uber.org/zap"
@@ -294,3 +295,13 @@ func (s *MessageService) InvalidateUserUnreadCache(userID, conversationID string
 	cache.InvalidateUnreadConversation(s.baseCache, userID)
 	s.conversationCache.InvalidateUnreadCount(userID, conversationID)
 }
+
+// GetMessagesByCursor 游标分页获取会话消息
+func (s *MessageService) GetMessagesByCursor(ctx context.Context, conversationID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Message], error) {
+	return s.messageRepo.GetMessagesByCursor(ctx, conversationID, req)
+}
+
+// GetConversationsByCursor 游标分页获取用户会话列表
+func (s *MessageService) GetConversationsByCursor(ctx context.Context, userID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Conversation], error) {
+	return s.messageRepo.GetConversationsByCursor(ctx, userID, req)
+}