perf: Sprint 2a — caching, parallel pipeline, async I/O, tests

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

fix(sprint-f2): fiabilité — error handling, async I/O, pagination, cache, prompts

fix(sprint-f1): sécurité — path traversal, SSRF, CORS, validation inputs

feat(sprint5): frontend React minimal — visionneuse + 4 couches

feat(sprint4-session-a): API FastAPI — structure de base + endpoints lecture et export