feat(amneziawg): add native AmneziaWG protocol backend

AmneziaWG (WireGuard plus DPI-resistant obfuscation) needs no Docker
here — it runs as a genuine kernel interface via awg-quick/awg, managed
the same way internal/mtproto manages mtg: one Inbound row is one
desired Instance, and a Manager reconciles running interfaces toward
the database every 10s (internal/web/job/amneziawg_job.go) plus
immediately after a client edit (applyLocalAmneziaWG).

Clients reuse model.Client verbatim (the same PrivateKey/PublicKey/
PreSharedKey/AllowedIPs fields WireGuard already uses), so bulk
operations, the QR/share-link modal and subscriptions come from the
shared inbound infrastructure instead of a parallel implementation.
internal/amneziawg owns the obfuscation param generator/validator
(ported from coinman-dev/3ax-ui, upgraded to AmneziaWG 2.0's S3/S4
padding and I1 signature packet) and the exec wrapper around
awg-quick/awg, with fingerprint-based reconcile (noop / reload-via-
syncconf / full restart) mirroring mtproto.Manager so a same-protocol
edit doesn't force an unnecessary interface bounce that would drop
every peer's connection.

Frontend and install.sh's DKMS/awg-tools setup are tracked separately;
this is backend-only.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/database/model/model.go

@@ -32,6 +32,7 @@ const (
 	WireGuard   Protocol = "wireguard"
 	Hysteria    Protocol = "hysteria"
 	MTProto     Protocol = "mtproto"
+	AmneziaWG   Protocol = "amneziawg"
 )
 
 // User represents a user account in the 3x-ui panel.
@@ -60,7 +61,7 @@ type Inbound struct {
 	// Xray configuration fields
 	Listen            string   `json:"listen" form:"listen"`
 	Port              int      `json:"port" form:"port" validate:"gte=0,lte=65535" example:"443"`
-	Protocol          Protocol `json:"protocol" form:"protocol" validate:"required,oneof=vmess vless trojan shadowsocks wireguard hysteria http mixed tunnel tun mtproto" example:"vless"`
+	Protocol          Protocol `json:"protocol" form:"protocol" validate:"required,oneof=vmess vless trojan shadowsocks wireguard hysteria http mixed tunnel tun mtproto amneziawg" example:"vless"`
 	Settings          string   `json:"settings" form:"settings"`
 	StreamSettings    string   `json:"streamSettings" form:"streamSettings"`
 	Tag               string   `json:"tag" form:"tag" gorm:"unique" example:"in-443-tcp"`