feat: project skeleton + DB migrations V1-V12 + ga-common framework

- Gradle multi-module: ga-common, ga-core, ga-api, ga-batch, ga-admin
- Flyway V1-V12: org/product/rule/receive/ledger/settle/batch/code/menu/system + seed
- ga-common (complete):
  - model: ApiResponse, PageResponse, SearchParam, TreeNode
  - exception: ErrorCode, BizException, GlobalExceptionHandler
  - annotation + aop: @RequirePermission, @DataChangeLog, ApiLog
  - auth + security: JWT, SecurityConfig
  - mybatis: BaseMapper, EncryptTypeHandler, JsonTypeHandler, AuditInterceptor
  - code (Redis cached) / menu (RBAC tree) / file / system / notification
  - excel: SXSSF+Cursor export, SAX+batch import (1M/700K rows)
  - util: Date/Money/Mask/Encrypt/Security

ga-common/src/main/java/com/ga/common/auth/LoginUser.java

@@ -0,0 +1,36 @@
+package com.ga.common.auth;
+
+import lombok.Builder;
+import lombok.Getter;
+import org.springframework.security.core.GrantedAuthority;
+import org.springframework.security.core.authority.SimpleGrantedAuthority;
+import org.springframework.security.core.userdetails.UserDetails;
+
+import java.util.Collection;
+import java.util.List;
+
+/**
+ * SecurityContext 에 들어갈 인증 주체.
+ */
+@Getter
+@Builder
+public class LoginUser implements UserDetails {
+
+    private final Long userId;
+    private final String loginId;
+    private final String userName;
+    private final Long agentId;
+    private final List<String> roleCodes;
+
+    @Override public Collection<? extends GrantedAuthority> getAuthorities() {
+        return roleCodes == null ? List.of() :
+                roleCodes.stream().<GrantedAuthority>map(r -> new SimpleGrantedAuthority("ROLE_" + r)).toList();
+    }
+
+    @Override public String getPassword() { return null; }
+    @Override public String getUsername() { return loginId; }
+    @Override public boolean isAccountNonExpired()     { return true; }
+    @Override public boolean isAccountNonLocked()      { return true; }
+    @Override public boolean isCredentialsNonExpired() { return true; }
+    @Override public boolean isEnabled()               { return true; }
+}