fix: comprehensive repo audit — 15 issues fixed

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

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

fix(sprint-f0): fondations — lazy imports, schémas conformes, pydantic-settings

feat(sprint4-session-c): branchement pipeline réel sur les jobs + tests job_runner