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/reverse.md

@@ -132,8 +132,10 @@ view's A and AAAA records use. An address with several names gets one PTR:
 3. the shortest name, then the first alphabetically.
 
 Jarl says which name it chose, and which it didn't, so you can change it.
-A reverse zone's page shows how many addresses still lack a PTR, with a
-button to add them. `jarl zone show` shows it as `Missing PTRs`.
+A reverse zone's page shows the addresses that still lack a PTR record and
+the name each would get (the first 25), with a button to add them. `jarl
+zone show` shows how many as `Missing PTRs`; the API lists them in
+`ptr_missing_list`.
 
 ## Keeping PTR records in step