When a cell is connected to others, changing the local WireGuard address
or Docker ip_range to a subnet that overlaps a connected cell's vpn_subnet
would break routing. Both now return 409 with the conflicting cell name.
- wireguard.address: derive network from new address, check all connected
cells' vpn_subnet for overlap (after existing format validation)
- ip_range: check all connected cells' vpn_subnet for overlap (after
existing RFC-1918 validation)
Tests: 4 cases each (overlap → 409, no overlap → ok, no cells → ok,
format error still fires first → 400).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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