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/V012__AddTestAdmin.sql

@@ -1,6 +1,6 @@
--- 테스트 계정 추가 (E2E 자동 테스트용)
--- 비밀번호: test123456
--- bcrypt hash: $2a$11$VKz.3zR0QFGZxJZQJ/M6w.3XjfQTp5emXyqhT3jrDZWCqYIxJkAOq
+-- 테스트 계정 추가 (E2E Playwright 자동 테스트용)
+-- 비밀번호: TestAdmin@123456
+-- API /api/auth/reset-password로 설정 (마이그레이션에서는 초기 해시만 설정)
 
 INSERT INTO admin_users (username, password_hash, created_at)
 VALUES ('test_admin', '$2a$11$VKz.3zR0QFGZxJZQJ/M6w.3XjfQTp5emXyqhT3jrDZWCqYIxJkAOq', NOW())