The catch-all, the dashboard and the hive-domain vhost were built in
vhosts.nix with the listen set, TLS attrs and headers applied by hand.
They are declared under services.hyperhive.gateway.vhosts now, so every
vhost behind the gateway goes through declared-vhosts.nix.
Two gateway-side rules cover what the catch-all needed: a vhost can set
securityHeaders = false (the catch-all only returns 444), and `default`
is rendered at mkDefault priority so an operator's own default vhost
still wins. The matrix discovery locations are unchanged.
The rendered nginx vhosts are unchanged.
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