Initialize RTGS prototype repository

3센터 A-A-A RTGS 실시간총액결제 프로토타입 최초 버전관리 시작.
- backend: Kotlin/Gradle 멀티모듈(sequencer, common, 채널/센터 모듈)
- frontend: Vite + TS
- infra: docker-compose, prometheus/grafana, ELK 네이티브 스크립트
- loadtest(k6), sim(장애/순서/정합성 시나리오), docs(PoC 보고서/복원력방안)

backend/build.gradle.kts

@@ -0,0 +1,37 @@
+plugins {
+    // Applied per-subproject; declared here (apply false) so versions are shared.
+    kotlin("jvm") version "2.0.20" apply false
+    kotlin("plugin.spring") version "2.0.20" apply false
+    kotlin("plugin.jpa") version "2.0.20" apply false
+    id("org.springframework.boot") version "3.3.4" apply false
+    id("io.spring.dependency-management") version "1.1.6" apply false
+}
+
+allprojects {
+    group = "kr.or.bok.rtgs"
+    version = "0.1.0"
+
+    repositories {
+        mavenCentral()
+    }
+}
+
+subprojects {
+    apply(plugin = "org.jetbrains.kotlin.jvm")
+
+    // All modules target Java 21 bytecode (cloud parity: Temurin 21).
+    the<JavaPluginExtension>().toolchain {
+        languageVersion.set(JavaLanguageVersion.of(21))
+    }
+
+    tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {
+        compilerOptions {
+            freeCompilerArgs.add("-Xjsr305=strict")
+            jvmTarget.set(org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_21)
+        }
+    }
+
+    tasks.withType<Test>().configureEach {
+        useJUnitPlatform()
+    }
+}