feature/add german national lib

[codebude]

• Add Deutsche Nationalbibliothek as book import source
• Add Deutsche Nationalbibliothek to import search frontend
• Document Deutsche Nationalbibliothek import source

[github-actions]

🧪 Test Results — bc04722

✅ Backend (pytest) — 1175 passed, 0 failed, 0 skipped (1175 total)
✅ CLI (pytest) — 54 passed, 0 failed, 0 skipped (54 total)
✅ Frontend (vitest) — 543 passed, 0 failed, 0 skipped (543 total)
✅ E2E (Playwright) — 79 passed, 0 failed, 0 skipped (79 total)

Summary: ✅ All 1851 tests passed

[mj0nez]

Nice, works as expected! Only covers are not yet fetched. :)