nix: split hive-forge into service and deploy-mode files

`swarm.forge` (what the forge is to every hive: ports, domain, public and
root URLs, OIDC client id and callback) moves to
nix/host-modules/hive-forge/service.nix, together with the rename of the
old `services.hyperhive.forge` tree and the removed `swarm.forge.sso.enable`,
both of which name `swarm.forge` paths. Everything else -- the
`deploy.forgejo` options, the whole `config` block including
`containers.hive-forge`, and the helpers only they read -- stays in
nix/host-modules/hive-forge/default.nix, which now imports ./service.nix.
Importing it from the directory's own default.nix, as hive-c0re/ and
hive-gateway/ do with their option files, keeps the flake's standalone
`nixosModules.hive-forge` export whole.

Both halves read `cfg`, `gatewayCfg`, `swarmDomain` and `deployCfg`. They
are option reads, so each file binds them from `config`. `ssoSourceName`,
`defaultRootUrl` and `effectiveRootUrl` are not options and both halves
need them (the service half builds `sso.redirectUri` from them, the deploy
half registers the login source and sets ROOT_URL), so they are duplicated,
with a note at each copy. `ssoRedirectUri` is read only by the service
half and moves.

`swarm.forge.publicUrl` and `swarm.forge.sso.redirectUri` default from
`deploy.forgejo.behindGateway`; both move as they are.

A pure move: option paths, option definitions and config are unchanged
apart from comments: the two on either side of the cut, the
`ssoRedirectUri` comment and the duplication notes, and the rename
precedent in ./deploy.nix, which now names ./hive-forge/service.nix.

Refs #3742

nix/host-modules/deploy.nix

@@ -36,7 +36,7 @@ in
   imports = [
     # Same type, same meaning, new path — so a rename carries it exactly
     # and existing configs keep evaluating with one warning naming both
-    # paths. Precedent: ./hive-forge/default.nix, ./hive-matrix.nix.
+    # paths. Precedent: ./hive-forge/service.nix, ./hive-matrix.nix.
     (lib.mkRenamedOptionModule
       [ "services" "hyperhive" "swarm" "grafana" "enable" ]
       [ "services" "hyperhive" "deploy" "grafana" "enable" ]