feat: add ai gateway foundation

backend/internal/ai/gateway.go

@@ -0,0 +1,45 @@
+package ai
+
+import (
+	"errors"
+
+	"gorm.io/gorm"
+	"senlinai-agent/backend/internal/domain"
+)
+
+type Gateway struct {
+	db        *gorm.DB
+	systemKey string
+}
+
+type SelectedKey struct {
+	Provider string
+	APIKey   string
+	KeyType  string
+}
+
+func NewGateway(database *gorm.DB, systemKey string) *Gateway {
+	return &Gateway{db: database, systemKey: systemKey}
+}
+
+func (g *Gateway) SelectKey(userID uint) (SelectedKey, error) {
+	var userKey domain.AIKey
+	if err := g.db.Where("user_id = ?", userID).First(&userKey).Error; err == nil {
+		return SelectedKey{Provider: userKey.Provider, APIKey: userKey.EncryptedAPIKey, KeyType: "user"}, nil
+	}
+	if g.systemKey == "" {
+		return SelectedKey{}, errors.New("no ai key available")
+	}
+	return SelectedKey{Provider: "openai", APIKey: g.systemKey, KeyType: "system"}, nil
+}
+
+func (g *Gateway) RecordCall(userID uint, provider string, usedKeyType string, action string, status string, errText string) error {
+	return g.db.Create(&domain.AICallLog{
+		UserID:      userID,
+		Provider:    provider,
+		UsedKeyType: usedKeyType,
+		Action:      action,
+		Status:      status,
+		Error:       errText,
+	}).Error
+}