auth: directory leavers check, dashboard warning, Samba AD check (S5)

Spec 12 S5: job directoryCheck (hourly) looks up every active user with a directory account;
one deleted, disabled there or no longer matched by the user filter is disabled here, sessions
ended. Nothing changes when the directory is unreachable or a run would disable most people;
the last admin with a password is kept; nobody is re-enabled. Dashboard warning
directory_check (system.directoryWarning). The by-id search is a filter object: ldapts string
filters send bytes >= 0x80 as UTF-8, so AD's objectGUID never matched. Live test against a
Samba 4 AD DC: nested-group admin, the disabled bit, objectGUID, a deleted user disabled.

docs/spec/04-backend-api.md

@@ -425,7 +425,7 @@ As built (M11-T6): `routes/reports.ts`. `from`/`to` are company-time-zone days,
 | GET/PATCH | `/api/settings` | whole settings object; PATCH merges known keys only. `smtp` comes back as `{host, port, secure, username, from, hasPassword}` and is written with an optional `password` (empty keeps the stored one). `system.*`, `sms.webhookToken` and `setup.completed` are read-only. References (DIDs, extensions, destinations) are checked, and a transport a trunk uses can't be turned off |
 | POST | `/api/settings/smtp/test` | `{to}` sends a test email |
 | POST | `/api/settings/emergency/test` | runs 04 §5.3 steps 2–4 with `TEST` in front (email, ring-out, banner); records nothing and dials nothing (06 §6.5). Returns `{notified}` |
-| GET | `/api/system/warnings` | `{items: [{code, message, link}]}` for the dashboard banner: `restart_required`, `e911_cid_missing` (a trunk exists, no default E911 caller ID), `e911_address_unconfirmed` (that DID has no `e911_address`), `smtp_missing`, `tls_cert` (TLS on and the cert missing or expiring within 14 days, M11-T2), `backup_old` (backups on and today's not made yet though the job had its chance, M11-T3). "Trunk down" comes from live state in the browser |
+| GET | `/api/system/warnings` | `{items: [{code, message, link}]}` for the dashboard banner: `restart_required`, `e911_cid_missing` (a trunk exists, no default E911 caller ID), `e911_address_unconfirmed` (that DID has no `e911_address`), `smtp_missing`, `tls_cert` (TLS on and the cert missing or expiring within 14 days, M11-T2), `backup_old` (backups on and today's not made yet though the job had its chance, M11-T3), `directory_check` (12 §4.2, §4.5: the directory unreachable at a sign-in or by `directoryCheck`, or that job refusing to disable most people). "Trunk down" comes from live state in the browser |
 | POST | `/api/settings/network/detect-public-ip` | |
 | GET | `/api/system/status` | `{ db: ok, ami: connected, asteriskVersion, uptime, trunks: [...], disk: {free, total}, restartRequired, lastGeneratorRun }` |
 | GET | `/api/system/tls` | the SIP TLS certificate in `TLS_DIR`: `{ok: true, subject, names, notAfter}` or `{ok: false, error}` (M11-T2) |