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.

internal/cmd/version.go

@@ -1,4 +1,4 @@
 package cmd
 
 // Version is the CLI version. Overridden by goreleaser ldflags at release time.
-var Version = "0.8.1"
+var Version = "0.9.0"