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/pkg/cursor/cursor.go

@@ -0,0 +1,152 @@
+package cursor
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+)
+
+// SortOrder 游标排序类型
+type SortOrder int
+
+const (
+	// SortByCreatedAtDesc 按创建时间降序
+	SortByCreatedAtDesc SortOrder = iota
+	// SortByCreatedAtAsc 按创建时间升序
+	SortByCreatedAtAsc
+	// SortByIDDesc 按ID降序
+	SortByIDDesc
+	// SortByIDAsc 按ID升序
+	SortByIDAsc
+	// SortBySeqDesc 按序列号降序（用于消息）
+	SortBySeqDesc
+	// SortBySeqAsc 按序列号升序
+	SortBySeqAsc
+	// SortByUpdatedAtDesc 按更新时间降序
+	SortByUpdatedAtDesc
+	// SortByUpdatedAtAsc 按更新时间升序
+	SortByUpdatedAtAsc
+	// SortByJoinTimeDesc 按加入时间降序（用于群成员）
+	SortByJoinTimeDesc
+	// SortByJoinTimeAsc 按加入时间升序（用于群成员）
+	SortByJoinTimeAsc
+)
+
+// String 返回排序类型的字符串表示
+func (s SortOrder) String() string {
+	switch s {
+	case SortByCreatedAtDesc:
+		return "created_at DESC"
+	case SortByCreatedAtAsc:
+		return "created_at ASC"
+	case SortByIDDesc:
+		return "id DESC"
+	case SortByIDAsc:
+		return "id ASC"
+	case SortBySeqDesc:
+		return "seq DESC"
+	case SortBySeqAsc:
+		return "seq ASC"
+	case SortByUpdatedAtDesc:
+		return "updated_at DESC"
+	case SortByUpdatedAtAsc:
+		return "updated_at ASC"
+	case SortByJoinTimeDesc:
+		return "join_time DESC"
+	case SortByJoinTimeAsc:
+		return "join_time ASC"
+	default:
+		return "created_at DESC"
+	}
+}
+
+// IsDescending 是否为降序
+func (s SortOrder) IsDescending() bool {
+	return s == SortByCreatedAtDesc || s == SortByIDDesc || s == SortBySeqDesc || s == SortByUpdatedAtDesc || s == SortByJoinTimeDesc
+}
+
+// SortField 返回排序字段名
+func (s SortOrder) SortField() string {
+	switch s {
+	case SortByCreatedAtDesc, SortByCreatedAtAsc:
+		return "created_at"
+	case SortByIDDesc, SortByIDAsc:
+		return "id"
+	case SortBySeqDesc, SortBySeqAsc:
+		return "seq"
+	case SortByUpdatedAtDesc, SortByUpdatedAtAsc:
+		return "updated_at"
+	case SortByJoinTimeDesc, SortByJoinTimeAsc:
+		return "join_time"
+	default:
+		return "created_at"
+	}
+}
+
+// cursorJSON 游标 JSON 结构（用于序列化）
+type cursorJSON struct {
+	SortValue string    `json:"sv"` // 排序字段值（时间戳或ID）
+	ID        string    `json:"id"` // 记录唯一ID
+	Order     SortOrder `json:"so"` // 排序方式
+}
+
+// Cursor 游标数据结构
+type Cursor struct {
+	SortValue string    // 排序字段值（时间戳或ID）
+	ID        string    // 记录唯一ID
+	Order     SortOrder // 排序方式
+}
+
+// Encode 编码游标为 Base64URL 字符串
+func (c *Cursor) Encode() string {
+	if c == nil {
+		return ""
+	}
+
+	data, err := json.Marshal(&cursorJSON{
+		SortValue: c.SortValue,
+		ID:        c.ID,
+		Order:     c.Order,
+	})
+	if err != nil {
+		return ""
+	}
+
+	return base64.URLEncoding.EncodeToString(data)
+}
+
+// DecodeCursor 解码 Base64URL 字符串为游标
+func DecodeCursor(encoded string) (*Cursor, error) {
+	if encoded == "" {
+		return nil, nil
+	}
+
+	data, err := base64.URLEncoding.DecodeString(encoded)
+	if err != nil {
+		// 尝试使用 RawURLEncoding（不带填充的编码）
+		data, err = base64.RawURLEncoding.DecodeString(encoded)
+		if err != nil {
+			return nil, errors.New("invalid cursor encoding: " + err.Error())
+		}
+	}
+
+	var cj cursorJSON
+	if err := json.Unmarshal(data, &cj); err != nil {
+		return nil, errors.New("invalid cursor format: " + err.Error())
+	}
+
+	return &Cursor{
+		SortValue: cj.SortValue,
+		ID:        cj.ID,
+		Order:     cj.Order,
+	}, nil
+}
+
+// NewCursor 创建新的游标
+func NewCursor(sortValue, id string, order SortOrder) *Cursor {
+	return &Cursor{
+		SortValue: sortValue,
+		ID:        id,
+		Order:     order,
+	}
+}