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/message_repo.go

@@ -2,8 +2,10 @@ package repository
 
 import (
 	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/cursor"
 	"context"
 	"fmt"
+	"strconv"
 	"strings"
 	"time"
 
@@ -655,3 +657,125 @@ func (r *MessageRepository) UpdateConversationLastSeqWithTx(tx *gorm.DB, convID
 			"updated_at":    time.Now(),
 		}).Error
 }
+
+// ========== Cursor Pagination Methods ==========
+
+// GetMessagesByCursor 游标分页获取会话消息
+// 消息按 seq DESC 排序
+func (r *MessageRepository) GetMessagesByCursor(ctx context.Context, conversationID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Message], error) {
+	// 构建基础查询
+	query := r.db.WithContext(ctx).Model(&model.Message{}).Where("conversation_id = ?", conversationID)
+
+	// 使用游标构建器，消息按 seq DESC 排序
+	builder := cursor.NewBuilder(query, cursor.SortBySeqDesc).
+		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.Message]([]*model.Message{}, "", "", false), nil
+	}
+
+	// 执行查询
+	var messages []*model.Message
+	if err := builder.Build().Find(&messages).Error; err != nil {
+		return nil, err
+	}
+
+	// 解密消息
+	if len(messages) > 0 {
+		model.BatchDecryptMessagesParallel(messages)
+	}
+
+	// 构建响应
+	pageSize := builder.GetPageSize()
+	hasMore := cursor.HasMore(len(messages), pageSize)
+	if hasMore {
+		messages = messages[:pageSize]
+	}
+
+	// 生成游标
+	var nextCursor, prevCursor string
+	if len(messages) > 0 {
+		// 下一页游标
+		if hasMore {
+			lastMsg := messages[len(messages)-1]
+			nextCursor = cursor.NewCursor(
+				strconv.FormatInt(lastMsg.Seq, 10),
+				lastMsg.ID,
+				cursor.SortBySeqDesc,
+			).Encode()
+		}
+		// 上一页游标
+		firstMsg := messages[0]
+		prevCursor = cursor.NewCursor(
+			strconv.FormatInt(firstMsg.Seq, 10),
+			firstMsg.ID,
+			cursor.SortBySeqDesc,
+		).Encode()
+	}
+
+	return cursor.NewCursorPageResult(messages, nextCursor, prevCursor, hasMore), nil
+}
+
+// GetConversationsByCursor 游标分页获取用户会话列表
+// 会话按置顶优先，然后按 updated_at DESC 排序
+// 注意：会话列表的排序涉及 conversation_participants 表的 is_pinned 和 updated_at 字段
+func (r *MessageRepository) GetConversationsByCursor(ctx context.Context, userID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Conversation], error) {
+	// 构建基础查询 - 关联 conversation_participants 表
+	query := r.db.WithContext(ctx).Model(&model.Conversation{}).
+		Joins("INNER JOIN conversation_participants cp ON conversations.id = cp.conversation_id").
+		Where("cp.user_id = ? AND cp.hidden_at IS NULL", userID).
+		Preload("Group")
+
+	// 会话列表需要特殊处理：先按置顶排序，再按更新时间排序
+	// 由于涉及多字段排序（is_pinned DESC, updated_at DESC），需要自定义处理
+	builder := cursor.NewBuilder(query, cursor.SortByUpdatedAtDesc).
+		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.Conversation]([]*model.Conversation{}, "", "", false), nil
+	}
+
+	// 执行查询 - 需要添加置顶排序
+	var conversations []*model.Conversation
+	query = builder.Build()
+	// 添加置顶排序（在游标排序之前）
+	query = query.Order("cp.is_pinned DESC").Order("conversations.updated_at DESC")
+
+	if err := query.Find(&conversations).Error; err != nil {
+		return nil, err
+	}
+
+	// 构建响应
+	pageSize := builder.GetPageSize()
+	hasMore := cursor.HasMore(len(conversations), pageSize)
+	if hasMore {
+		conversations = conversations[:pageSize]
+	}
+
+	// 生成游标
+	var nextCursor, prevCursor string
+	if len(conversations) > 0 {
+		// 下一页游标
+		if hasMore {
+			lastConv := conversations[len(conversations)-1]
+			nextCursor = cursor.NewCursor(
+				cursor.FormatTime(lastConv.UpdatedAt),
+				lastConv.ID,
+				cursor.SortByUpdatedAtDesc,
+			).Encode()
+		}
+		// 上一页游标
+		firstConv := conversations[0]
+		prevCursor = cursor.NewCursor(
+			cursor.FormatTime(firstConv.UpdatedAt),
+			firstConv.ID,
+			cursor.SortByUpdatedAtDesc,
+		).Encode()
+	}
+
+	return cursor.NewCursorPageResult(conversations, nextCursor, prevCursor, hasMore), nil
+}