Certificate export, missing PTR list, simpler "Used for"

- Certificates can be exported for use outside Jarl: Export… on the
  Certificates page (download or copy), jarl cert export, and
  GET /certificates/{name}/pem?part=fullchain|cert|key|bundle.
  Administrators only; key exports are audited.
- A reverse zone's missing-PTR banner lists the addresses and names
  (ptr_missing_list in the zone API).
- "Used for" shows DoT/DoH for named, and "not used by Jarl" instead of
  "nothing yet".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/certificates.md

@@ -216,11 +216,35 @@ and the last good one stays in use. jarld runs as user `jarl`, which must
 be able to read both files (e.g. `chgrp jarl` and `chmod 640`, or have
 your renewal hook copy them somewhere it can read).
 
+## Using a certificate outside Jarl
+
+A certificate needn't be used by Jarl at all ("not used by Jarl" on the
+Certificates page): Jarl obtains and renews it, and you export it for
+another program, such as a mail server or a reverse proxy. **Export…** on
+the Certificates page downloads each file or copies it to the clipboard
+(copying needs the web UI over HTTPS, or from localhost). Or:
+
+```sh
+jarl cert export mail --dir /etc/ssl/mail             # fullchain.pem, and privkey.pem (mode 600)
+jarl cert export mail --part bundle > /etc/haproxy/mail.pem
+```
+
+The parts are `fullchain` (the certificate and its chain), `cert` (the
+certificate alone), `key` and `bundle` (the key, then the chain, in one
+file, as HAProxy wants). Only administrators can export, and every export
+of the key is in the audit log (`certificate.export_key`).
+
+Jarl renews the certificate, not your copy: export it again after each
+renewal (`jarl cert show` says when that is), for example from a daily
+cron job. A program on this server can instead read the files Jarl writes
+for named (use `dns`), which Jarl keeps current.
+
 ## Security notes
 
 - Certificates Jarl obtains, their keys and the ACME account key are kept
   in Jarl's database, so they are in its backups (like TSIG secrets). The
-  API never returns a key.
+  API returns a key only to an administrator who exports it, and records
+  that in the audit log.
 - Every publicly trusted certificate is published in Certificate
   Transparency logs, with all its names. A name like `ns1.adbraun.lol`
   used for both the web UI and DNS reveals less than separate names. Avoid
@@ -240,6 +264,7 @@ your renewal hook copy them somewhere it can read).
 | GET, PATCH, DELETE | `/certificates/{name}` | `domains` and `uses` replace the whole list |
 | POST | `/certificates/{name}/issue` | obtain now: 202, then poll `GET /certificates/{name}` until `issuing_since` is gone; a file certificate is reread (200) |
 | POST | `/certificates/{name}/check` | the routes for an existing certificate |
+| GET | `/certificates/{name}/pem?part=fullchain` | the certificate as PEM, to save: `fullchain` (default), `cert`, `key` or `bundle` (key and chain). Exports of the key are audited |
 
 ## Testing with Pebble