feat(admin-console): Stage 4 — games & runtimes domain

Add the games, runtime, and engine-version pages over the existing lobby,
runtime, and engine-version services (no new business logic).

- GET/POST /_gm/games                         list + create public game
- GET      /_gm/games/{id}                    detail incl. runtime snapshot
- POST     /_gm/games/{id}/force-start|stop    game state actions
- POST     /_gm/games/{id}/ban-member          ban a member (uuid + reason)
- POST     /_gm/games/{id}/runtime/restart|patch|force-next-turn
- GET/POST /_gm/engine-versions               registry + register
- POST     /_gm/engine-versions/{ver}/disable disable a version

Console depends on GameAdmin / RuntimeAdmin / EngineVersionAdmin interfaces
(satisfied by the concrete services) so the pages render in tests without a
database. Collection-mutating POSTs are mounted on the collection path to avoid
a static-vs-param route conflict in gin. Writes flow through the CSRF guard and
redirect back; the create form parses datetime-local as UTC.

Tests: list/detail (with and without a runtime), create (visibility/owner/time
assertions), force-start (+ bad-CSRF), ban-member (+ bad uuid), runtime patch
(+ missing version), engine-version list/register/disable, and unavailable.

Docs: backend/docs/admin-console.md page inventory extended.

backend/internal/adminconsole/assets/console.css

@@ -98,3 +98,5 @@ button {
 button:hover { filter: brightness(1.1); }
 button.danger { background: var(--danger); color: #1a0606; }
 code { background: var(--bg); padding: 0.05rem 0.3rem; border-radius: 4px; }
+.actions { display: flex; flex-wrap: wrap; gap: 0.6rem; margin: 0.8rem 0; }
+.actions form { margin: 0; }