feat: game templates, schemas and validator

Public game templates for GBR Hosting, one games/<id>/game.yaml per game,
imported from the panel. A Go tool validates them against JSON Schemas and
checks for credentials, private addresses and consents that default to
accepted; CI runs it on every push.

Admin/RCON passwords are no longer shared defaults: they are blank with
x-generate so the panel creates one per server, and Palworld/ARK no longer
publish their RCON/REST ports.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go.mod

@@ -0,0 +1,10 @@
+module www.gitea.gbreed.com/gbreed-public/gbr-games
+
+go 1.24
+
+require (
+	github.com/santhosh-tekuri/jsonschema/v6 v6.0.3
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require golang.org/x/text v0.14.0 // indirect