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/annotation/ExcelColumn.java

@@ -0,0 +1,22 @@
+package com.ga.common.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * 엑셀 컬럼 매핑 어노테이션. ExcelExporter / ExcelImporter 가 사용.
+ */
+@Target(ElementType.FIELD)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface ExcelColumn {
+    String header();
+    int order();
+    int width() default 15;
+    String format() default "";
+    String codeGroup() default "";
+    boolean masked() default false;
+    String defaultValue() default "";
+    boolean required() default false;
+}