refactor backend packages and global db service

backend/internal/models/ai_key.go

@@ -0,0 +1,16 @@
+package models
+
+import "time"
+
+type SenlinAgentAIKey struct {
+	ID              uint   `gorm:"primaryKey"`
+	UserID          uint   `gorm:"uniqueIndex;not null"`
+	Provider        string `gorm:"not null"`
+	EncryptedAPIKey string `gorm:"not null"`
+	CreatedAt       time.Time
+	UpdatedAt       time.Time
+}
+
+func (SenlinAgentAIKey) TableName() string {
+	return "senlin_agent_ai_keys"
+}