Moves `forgeVhost` out of the gateway's vhosts.nix and the forge's
`address=` rule out of dnsmasq.nix, into nix/host-modules/hive-forge —
the module that already owns everything else about the forge.
The gateway keeps what is gateway knowledge (the listen set, which
issuer covers a name, the header block) and loses the last reason it
had to read `swarm.forge` at all: `forgeCfg` is gone from both files
and from the module's `let`.
Both halves stay gated on `behindGateway` — with it off the operator
fronts forgejo themselves, so this hive must neither claim the vhost nor
answer DNS for the name.
d60a0585d6