feat(serials): preserve stubbed chapters

[audemed44]

Summary

• preserve local chapter numbering and content when upstream chapters are stubbed
• add stubbed/live chapter metadata plus partial volume reporting in the API and UI
• migrate existing serials by backfilling source keys and repairing preserved totals

Testing

• cd frontend && npx eslint src --ext js,jsx,ts,tsx --fix
• cd frontend && npx tsc --noEmit
• cd frontend && npm test
• cd backend && ./.venv/bin/python -m pytest tests/test_serial_service.py
• cd backend && ./.venv/bin/python -m pytest --cov=app --cov-fail-under=90