fix(backend): patch retrain pipeline, admin error handling, PatientVisit import

feat: complete remaining roadmap features (1.3, 1.5, 1.6, 3.1, 4.3)

feat(active-learning): add review queue API (list/annotate/skip/stats)

feat(active-learning): add entropy-based uncertainty sampler

feat(active-learning): add active_learning package init