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>

cmd/jarld/main.go

@@ -318,6 +318,7 @@ func serveCmd() *cobra.Command {
 				EABKeyID: a.cfg.ACME.Account.EABKeyID, EABHMAC: a.cfg.ACME.Account.EABHMAC,
 				Resolvers: a.cfg.ACME.Resolvers, CABundle: a.cfg.ACME.CABundle, CheckPropagation: a.cfg.ACME.CheckPropagation,
 				Dir: a.cfg.CertDir(), Version: version.String(), Porkbun: pb, PorkbunAllows: a.cfg.Porkbun.Allows,
+				MaxPerUser: a.cfg.Certificates.MaxPerUser,
 			}, a.store, a.eng, a.log)
 			cm.Start(ctx)
 			if a.cfg.ACME.Account.EABHMAC != "" {