fix(audit): resolve 15 critical and major technical debt issues

fix: security hardening, real AI costs, traceId propagation & audit logging

feat: agentic platform — text-to-sql, pedagogy advisor, security hardening & performance

feat(admin-chat): make AI copilot a full super-admin via platform knowledge + agentic tools

feat(admin): UX non-tech complète + i18n 4 langues

feat(billing): refonte UX non-tech + enrichissement contexte IA

feat(billing): close 3 remaining wallet gaps from implementation plan

feat(billing): implement full wallet/ledger system with hard-stop enforcement

feat(billing): enforce AI credit limits across all surfaces

feat(billing): complete billing system, push notifications, and tech debt fixes