docs: update guidelines and test account configuration to reflect current API-first implementation

- Update E2E testing section with test_admin account details (TestAdmin@123456)
- Add comprehensive admin account management via API (reset-password endpoint)
- Update migration comments to reference API-based password setting
- Align E2E workflow with Green-Blue deployment support (Nginx routing)
- Add backup policy documentation (daily 02:00 AM, 30-day retention)
- Clarify test account isolation for repeatable E2E execution

Current Status:
✅ Phase 5: JWT token improvements (15m access + 7d refresh)
✅ Phase 7: API-First migration (9 Blazor pages, 6 controllers, 5 clients)
✅ Phase 6: SignalR notifications (stateless broadcast)
✅ Green-Blue deployment infrastructure (Nginx routing, configurable API port)
✅ Automated backups (daily PostgreSQL pg_dump)
✅ E2E testing with separate test_admin account

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

db/migrations/V013__EnsureAdminAndTestAccounts.sql

@@ -1,6 +1,6 @@
--- 관리자 계정 확실히 하기
--- admin: password = admin123
--- test_admin: password = test123456
+-- 관리자 계정 확실히 하기 (멱등성 보장)
+-- admin: 마이그레이션 V003에서 생성
+-- test_admin: 마이그레이션 V012에서 생성, API reset-password로 최종 설정
 
 -- V003에서 이미 생성된 admin 계정이 없으면 추가
 INSERT INTO admin_users (username, password_hash, created_at)