fix(model): accept tun protocol in inbound validation

Adding a TUN inbound failed with "request body failed validation" because the Inbound.Protocol oneof allowlist omitted "tun". Add it so the validator matches the protocol the frontend already offers.

Closes #4736

database/model/model.go

@@ -56,7 +56,7 @@ type Inbound struct {
 	// Xray configuration fields
 	Listen         string   `json:"listen" form:"listen"`
 	Port           int      `json:"port" form:"port" validate:"gte=1,lte=65535"`
-	Protocol       Protocol `json:"protocol" form:"protocol" validate:"required,oneof=vmess vless trojan shadowsocks wireguard hysteria http mixed tunnel"`
+	Protocol       Protocol `json:"protocol" form:"protocol" validate:"required,oneof=vmess vless trojan shadowsocks wireguard hysteria http mixed tunnel tun"`
 	Settings       string   `json:"settings" form:"settings"`
 	StreamSettings string   `json:"streamSettings" form:"streamSettings"`
 	Tag            string   `json:"tag" form:"tag" gorm:"unique"`