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>

internal/server/h_auth.go

@@ -444,6 +444,12 @@ func applyUserInput(u *model.User, in *api.UserInput) error {
 	if in.Comment != nil {
 		u.Comment = *in.Comment
 	}
+	if in.CertLimit != nil {
+		u.CertLimit = in.CertLimit
+		if *in.CertLimit < 0 {
+			u.CertLimit = nil
+		}
+	}
 	if in.Password != nil {
 		if *in.Password == "" {
 			u.PasswordHash = "" // service account: tokens only