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/cli.md

@@ -446,6 +446,8 @@ jarl cert show ns1
 jarl cert issue ns1                                  # renew now (--no-wait to return at once)
 jarl cert update ns1 --challenge porkbun             # auto, jarl or porkbun
 jarl cert update ns1 --use dns                       # --use and --domain replace the list (--use none)
+jarl cert export mail --dir /etc/ssl/mail            # fullchain.pem and privkey.pem, for another program
+jarl cert export mail --part bundle                  # one part to stdout: fullchain, cert, key, bundle
 jarl cert delete ns1
 jarl cert account                                    # the ACME account (set in jarl.yaml)
 ```