feat: 新增Chat会话和群聊API

- 新增chat_session相关模型、仓库和服务
- 新增chat_group相关模型、仓库和服务
- 新增session_handler和chat_group_handler
- 实现会话管理和群聊功能

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/internal/repository/chat_repository.go

@@ -0,0 +1,86 @@
+package repository
+
+import (
+	"x-agents/server/internal/model"
+
+	"gorm.io/gorm"
+)
+
+type ChatRepository struct {
+	db *gorm.DB
+}
+
+func NewChatRepository(db *gorm.DB) *ChatRepository {
+	return &ChatRepository{db: db}
+}
+
+// Session CRUD
+
+// CreateSession 创建会话
+func (r *ChatRepository) CreateSession(session *model.ChatSession) error {
+	return r.db.Create(session).Error
+}
+
+// GetSessionByID 根据ID获取会话
+func (r *ChatRepository) GetSessionByID(id string) (*model.ChatSession, error) {
+	var session model.ChatSession
+	err := r.db.Where("id = ?", id).First(&session).Error
+	if err != nil {
+		return nil, err
+	}
+	return &session, nil
+}
+
+// GetSessionsByUserID 获取用户的所有会话
+func (r *ChatRepository) GetSessionsByUserID(userID string, limit, offset int) ([]model.ChatSession, int64, error) {
+	var sessions []model.ChatSession
+	var total int64
+
+	query := r.db.Model(&model.ChatSession{}).Where("user_id = ?", userID)
+	query.Count(&total)
+
+	err := query.Order("updated_at DESC").Limit(limit).Offset(offset).Find(&sessions).Error
+	return sessions, total, err
+}
+
+// UpdateSession 更新会话
+func (r *ChatRepository) UpdateSession(session *model.ChatSession) error {
+	return r.db.Save(session).Error
+}
+
+// DeleteSession 删除会话
+func (r *ChatRepository) DeleteSession(id string) error {
+	// 先删除会话下的所有消息
+	r.db.Where("session_id = ?", id).Delete(&model.ChatMessage{})
+	// 再删除会话
+	return r.db.Delete(&model.ChatSession{}, "id = ?", id).Error
+}
+
+// Message CRUD
+
+// CreateMessage 创建消息
+func (r *ChatRepository) CreateMessage(message *model.ChatMessage) error {
+	return r.db.Create(message).Error
+}
+
+// GetMessagesBySessionID 获取会话的所有消息
+func (r *ChatRepository) GetMessagesBySessionID(sessionID string, limit, offset int) ([]model.ChatMessage, int64, error) {
+	var messages []model.ChatMessage
+	var total int64
+
+	query := r.db.Model(&model.ChatMessage{}).Where("session_id = ?", sessionID)
+	query.Count(&total)
+
+	err := query.Order("created_at ASC").Limit(limit).Offset(offset).Find(&messages).Error
+	return messages, total, err
+}
+
+// DeleteMessage 删除消息
+func (r *ChatRepository) DeleteMessage(id string) error {
+	return r.db.Delete(&model.ChatMessage{}, "id = ?", id).Error
+}
+
+// DeleteMessagesBySessionID 删除会话的所有消息
+func (r *ChatRepository) DeleteMessagesBySessionID(sessionID string) error {
+	return r.db.Where("session_id = ?", sessionID).Delete(&model.ChatMessage{}).Error
+}