chore: [API] logging, docs, seeder 등 부수 정리

- LOGICAL_RELATIONSHIPS.md 보완
- Legacy5130Calculator 수정
- logging.php 설정 추가
- KyungdongItemSeeder 수정
- docs/INDEX.md, changes 문서 경로 수정

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/seeders/Kyungdong/KyungdongItemSeeder.php

@@ -17,7 +17,7 @@
  * Phase 3.1: chandj.price_motor → items (SM) + prices 누락 품목
  * Phase 3.2: chandj.price_raw_materials → items (RM) + prices 누락 품목
  *
- * @see docs/plans/kd-items-migration-plan.md
+ * @see docs/dev_plans/kd-items-migration-plan.md
  */
 class KyungdongItemSeeder extends Seeder
 {