fix(security): restrict auth-config mutations to Admin role (#5)
Restrict manager-wide authentication configuration mutations (OIDC, SMTP, IP allowlist) to Admin role. Operators now receive 403 forbidden_role.
- New admin_required helper in settings.rs
- 4 gate changes: update_settings, discover_oidc, test_oidc, update_ip_whitelist
- 5 new AuditAction variants + migration 019
- SPA friendly error message on 403
- 3 admin_required unit tests pass (43/43)
- Full integration tests deferred to issue #15
Closes #5