fix: IIIF image fetch performance — retry with backoff, request size reduction, cursor warning

fix(backend): Sprint Fix 2 — data integrity, error handling, robustness

rename: Scriptorium AI → IIIF Studio across entire codebase

fix(sprint-f3): conformité spec — IIIF langue, ai_raw.json, prompts génériques

fix(ingest): add User-Agent and headers to httpx requests to fix Gallica 403

Sprint 2 Session B — Pipeline image : fetch IIIF + dérivé JPEG 1500px + thumbnail