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/common/src/main/resources/logback-spring.xml

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  RTGS 공통 로그 설정(전 서비스 공유). 콘솔 + Logstash(TCP :5000, JSON) 전송.
+  Logstash가 안 떠 있어도 TCP appender가 논블로킹으로 재연결하므로 서비스는 정상 동작.
+-->
+<configuration>
+    <include resource="org/springframework/boot/logging/logback/defaults.xml"/>
+
+    <springProperty scope="context" name="appName" source="spring.application.name" defaultValue="rtgs"/>
+    <springProperty scope="context" name="centerId" source="rtgs.center-id" defaultValue="DC1"/>
+
+    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>${CONSOLE_LOG_PATTERN}</pattern>
+            <charset>UTF-8</charset>
+        </encoder>
+    </appender>
+
+    <appender name="LOGSTASH" class="net.logstash.logback.appender.LogstashTcpSocketAppender">
+        <destination>localhost:5000</destination>
+        <keepAliveDuration>1 minute</keepAliveDuration>
+        <reconnectionDelay>5 second</reconnectionDelay>
+        <encoder class="net.logstash.logback.encoder.LogstashEncoder">
+            <customFields>{"service":"${appName}","center":"${centerId}"}</customFields>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="CONSOLE"/>
+        <appender-ref ref="LOGSTASH"/>
+    </root>
+</configuration>