feat: 新增Model管理模块

- 添加 Model 实体定义
- 实现 Model CRUD 接口
- 添加 Model 仓储层和服务层

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

server/internal/repository/model_repo.go

@@ -0,0 +1,53 @@
+package repository
+
+import (
+	"x-agents/server/internal/model"
+
+	"gorm.io/gorm"
+)
+
+// ModelRepository 模型仓储
+type ModelRepository struct {
+	db *gorm.DB
+}
+
+func NewModelRepository(db *gorm.DB) *ModelRepository {
+	return &ModelRepository{db: db}
+}
+
+// FindAll 获取所有模型
+func (r *ModelRepository) FindAll() ([]model.ModelInfo, error) {
+	var models []model.ModelInfo
+	err := r.db.Order("created_at desc").Find(&models).Error
+	return models, err
+}
+
+// FindByID 根据 ID 获取模型
+func (r *ModelRepository) FindByID(id string) (*model.ModelInfo, error) {
+	var model model.ModelInfo
+	err := r.db.Where("id = ?", id).First(&model).Error
+	if err != nil {
+		return nil, err
+	}
+	return &model, nil
+}
+
+// Create 创建模型
+func (r *ModelRepository) Create(info *model.ModelInfo) error {
+	return r.db.Create(info).Error
+}
+
+// Update 更新模型
+func (r *ModelRepository) Update(id string, info *model.ModelInfo) error {
+	return r.db.Where("id = ?", id).Updates(info).Error
+}
+
+// Delete 删除模型
+func (r *ModelRepository) Delete(id string) error {
+	return r.db.Where("id = ?", id).Delete(&model.ModelInfo{}).Error
+}
+
+// UpdateFields 更新指定字段
+func (r *ModelRepository) UpdateFields(id string, fields map[string]interface{}) error {
+	return r.db.Model(&model.ModelInfo{}).Where("id = ?", id).Updates(fields).Error
+}