vNext: Add multiple book import sources, configurable frontend upstream, and dependency upgrades

[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
• Update frontend and Python dependency lockfiles with minor safe/upgrades
• Migrate frontend imports to #lib and upgrade SvelteKit/Vitest
• Ignore intentional Pydantic deprecation warnings in backend tests
• Document Node.js 22.17 requirement and SvelteKit 3 upgrade
• Add Deutsche Nationalbibliothek as book import source
• Add Deutsche Nationalbibliothek to import search frontend
• Document Deutsche Nationalbibliothek import source
• feat: allow a configurable upstream for the frontend webserver
• docs: describe BACKEND_ADDR
• fix: use default.conf for config
• chore: addressed feedback
• Warn about invalid BACKEND_ADDR values
• Updated release notes for v1.10.0
• Fix release comparison links

[github-actions]

🧪 Test Results — 9d13b62

✅ 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