feature/add more search providers

[codebude]

• Add BnF, Inventaire and Library of Congress import sources to backend
• Add new search sources, localize import source names and report each search stage
• Document new import search sources and their per-source toggles
• Document new import sources and features in README
• Set keyed import sources off in example config and require credentials to enable them

[github-actions]

🧪 Test Results — db925d7

✅ Backend (pytest) — 1099 passed, 0 failed, 0 skipped (1099 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 1775 tests passed