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_group_repo.go

@@ -0,0 +1,56 @@
+package repository
+
+import (
+	"x-agents/server/internal/model"
+
+	"gorm.io/gorm"
+)
+
+// ChatGroupRepository 群聊仓储
+type ChatGroupRepository struct {
+	db *gorm.DB
+}
+
+// NewChatGroupRepository 创建群聊仓储
+func NewChatGroupRepository(db *gorm.DB) *ChatGroupRepository {
+	return &ChatGroupRepository{db: db}
+}
+
+// Create 创建群聊
+func (r *ChatGroupRepository) Create(group *model.ChatGroup) error {
+	return r.db.Create(group).Error
+}
+
+// FindByID 根据ID查询
+func (r *ChatGroupRepository) FindByID(id string) (*model.ChatGroup, error) {
+	var group model.ChatGroup
+	err := r.db.Where("id = ?", id).First(&group).Error
+	if err != nil {
+		return nil, err
+	}
+	return &group, nil
+}
+
+// FindByUserID 根据用户ID查询群聊列表
+func (r *ChatGroupRepository) FindByUserID(userID string, limit, offset int) ([]model.ChatGroup, int64, error) {
+	var groups []model.ChatGroup
+	query := r.db.Model(&model.ChatGroup{}).Where("user_id = ?", userID)
+
+	var total int64
+	if err := query.Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+
+	err := query.Order("created_at DESC").Limit(limit).Offset(offset).Find(&groups).Error
+	return groups, total, err
+}
+
+// Update 更新群聊
+func (r *ChatGroupRepository) Update(group *model.ChatGroup) error {
+	return r.db.Save(group).Error
+}
+
+// Delete 删除群聊
+func (r *ChatGroupRepository) Delete(id string) error {
+	return r.db.Delete(&model.ChatGroup{}, "id = ?", id).Error
+}