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>

games/minecraft-crossplay/test.yaml

@@ -0,0 +1,5 @@
+# yaml-language-server: $schema=../../schema/test.schema.json
+# Values CI uses to boot-test this game. Never put real credentials here: map them under 'secrets'.
+config:
+  EULA: true
+timeoutSec: 900