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-api/src/main/java/com/ga/api/GaApiApplication.java

@@ -0,0 +1,15 @@
+package com.ga.api;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+/**
+ * GA 수수료 정산 API 서버.
+ * 컴포넌트 스캔: com.ga 패키지 (common/core/api 모두 포함)
+ */
+@SpringBootApplication(scanBasePackages = {"com.ga"})
+public class GaApiApplication {
+    public static void main(String[] args) {
+        SpringApplication.run(GaApiApplication.class, args);
+    }
+}