docs(i18n): translate AUDIT_SECURITE_2026-05-15.md to English

Full English translation of the v1.9.0 security audit report
(31 findings LM2-01..LM2-31, methodology, cross-cutting analysis,
prioritized action plan, OWASP API Top 10 mapping).

The filename is intentionally kept as `AUDIT_SECURITE_2026-05-15.md`
(no rename) to preserve cross-references from CHANGELOG, README and
prior audits. A future cleanup pass can normalize to
`SECURITY_AUDIT_2026-05-15.md` if needed.

Note: code-level findings (e.g. `Identifiant d'espace invalide`) keep
their original French error strings verbatim where they reflect the
actual server response — auditor accuracy over translation purity.

No behavior change.

Author: chrlesur