Certificates for other hosts: jarl cert get

jarld obtains and renews a certificate for another host; the host asks
for it with one command, waits, downloads it when it is newer than its
copy and runs a hook with the paths in JARL_CERT_* variables.

- Certificates have an owner user; another user can't take the name.
  An existing one is obtained again only when a name is added or
  removed. A new certificates grant (names=) says which names a user
  may have; it is never part of "*". Users may own
  certificates.max_per_user certificates (10) or their own cert_limit.
- POST /certificates/{name}/request; owners may list, show, export and
  delete theirs; keys only over HTTPS, the socket or loopback; every
  download is recorded and shown as "Pulled by".
- CLI: jarl cert get (saved settings, all saved when no name), cert
  saved, cert forget, cert schedule (daily systemd timer, Linux, root).
- Migration 14. Tests: server, auth, and e2e against Pebble.

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

docs/configuration.md

@@ -101,6 +101,7 @@ jarld restarts.
 | `acme.ca_bundle` | `""` | PEM file of extra CA certificates to trust for the ACME server (a private CA). |
 | `acme.check_propagation` | `true` | Wait until every name server of the zone serves a challenge before the CA is told to look (they are asked directly, on port 53). |
 | `certificates.dir` | `<bind.config_dir>/tls` | Where certificates with the `dns` use are written for named: `<dir>/<name>/fullchain.pem` and `privkey.pem`. |
+| `certificates.max_per_user` | `10` | How many certificates a user may own through `jarl cert get` ([certificates.md](certificates.md#certificates-for-other-hosts)). A user's own limit (`jarl user update NAME --cert-limit N`) replaces it. |
 
 ### ACME (lego httpreq endpoint)