SuggestIPAllocation always looked up the legacy "wg0" server regardless
of which server the request was for, so any setup without a migrated
wg0 (i.e. every fresh multi-server install) failed with
"open db/servers/wg0.json: no such file or directory" when adding a
new client.
Now accepts an optional server_id query param (falling back to wg0 for
the legacy bare routes), and the per-server clients page passes its
own server ID.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
