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/api.md

@@ -604,6 +604,7 @@ Administrators only. See [certificates.md](certificates.md#api).
 | GET, PATCH, DELETE | `/certificates/{name}` | |
 | POST | `/certificates/{name}/issue` | 202: being obtained (poll until `issuing_since` is gone); a file certificate is reread (200) |
 | POST | `/certificates/{name}/check` | the routes of a certificate's names |
+| GET | `/certificates/{name}/pem?part=PART` | the certificate as PEM: `fullchain` (default), `cert`, `key`, `bundle`; key exports audited ([certificates.md](certificates.md#using-a-certificate-outside-jarl)) |
 
 `GET /status` includes `certificates` for administrators: `total`,
 `problems` (failing, expiring, expired), `next` and `next_after`,