Config: relay_url for a self-hosted iroh relay (RelayMode::Custom)

The sovereign hole-punch hook redoal's ADR-0021 milestone 3 needs: when
relay_url is set (VARDE_RELAY_URL or the config file), the endpoint
coordinates NAT traversal through the operator's own iroh relay instead
of n0's or none — takes precedence over wan_upload's n0 defaults. None
keeps the existing LAN-only / n0 behaviour, so nothing changes by default.

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

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