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>

.gitea/workflows/browser-e2e.yml

@@ -65,7 +65,7 @@ jobs:
           E2E_BASE_URL: http://${{ secrets.DEPLOY_HOST }}/taxbaik
           # E2E 테스트는 test_admin 테스트 계정 사용 (실 admin 계정과 분리)
           E2E_ADMIN_USERNAME: test_admin
-          E2E_ADMIN_PASSWORD: test123456
+          E2E_ADMIN_PASSWORD: TestAdmin@123456
         run: npm run test:e2e
 
       - name: Browser E2E summary