Migrate FightSystem modes and countdowns to GameCore

- Move shared arena-mode abstractions into GameCore
- Adapt fight countdowns and APIs to the shared model
- Add SteamWar issue audit workflow and SQL language settings

GameCore/build.gradle.kts

@@ -0,0 +1,22 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ */
+
+plugins {
+    steamwar.kotlin
+}
+
+dependencies {
+    compileOnly(libs.paperapi)
+    compileOnly(project(":KotlinCore", "default"))
+
+    testImplementation(libs.junit)
+    testImplementation(libs.hamcrest)
+}