feat(vpn): split-tunnel torrent traffic through managed WireGuard

In-process userspace WireGuard tunnel (wireguard-go + gVisor netstack) for
the managed-VPN add-on. No root, no OS routing changes: only the embedded
anacrolix/torrent client's peer + tracker traffic is routed through the
tunnel, so the swarm and trackers see the VPN IP, not the user's home IP.
unarr's control plane (API, heartbeats) keeps using the normal net.

- internal/vpn: FetchConfig (GET /api/internal/agent/vpn-config, Bearer auth,
  typed errors for disabled/not_provisioned/slot_on_device) + Up (parse .conf
  → uapi, CreateNetTUN, device Up) + DialContext/ListenPacket adapters.
- engine/torrent.go: when a tunnel is set, wire TrackerDialContext +
  HTTPDialContext + TrackerListenPacket to netstack, DisableUTP, and
  AddDialer(NetworkDialer{tcp, netstack}) for peer conns.
- config: downloads.vpn.enabled flag.
- daemon: bring up the tunnel before the torrent client; non-fatal on
  failure (logs + downloads in the clear); slot_on_device warns the user.
- version bump 0.8.1 → 0.9.0.

Pairs with the web VPN add-on (dormant behind NEXT_PUBLIC_VPN_ENABLED).
Runtime-verified once a VPNResellers trial provides a live endpoint.

go.mod

@@ -18,6 +18,7 @@ require (
 	github.com/torrentclaw/go-client v0.2.0
 	golang.org/x/term v0.41.0
 	golang.org/x/time v0.15.0
+	golang.zx2c4.com/wireguard v0.0.0-20250521234502-f333402bd9cb
 )
 
 require (
@@ -127,6 +128,8 @@ require (
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.35.0 // indirect
+	golang.zx2c4.com/wintun v0.0.0-20230126152724-0fa3db229ce2 // indirect
+	gvisor.dev/gvisor v0.0.0-20250503011706-39ed1f5ac29c // indirect
 	lukechampine.com/blake3 v1.4.1 // indirect
 	modernc.org/libc v1.70.0 // indirect
 	modernc.org/mathutil v1.7.1 // indirect