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/certs_test.go

@@ -45,7 +45,7 @@ func selfSigned(t *testing.T, dir string, names ...string) (certFile, keyFile st
 
 func withCerts(env *apiEnv) *certs.Manager {
 	quiet := slog.New(slog.NewTextHandler(io.Discard, nil))
-	m := certs.New(certs.Config{Dir: filepath.Join(env.t.TempDir(), "tls"), Directory: "https://acme.invalid/dir"}, env.st, env.srv.eng, quiet)
+	m := certs.New(certs.Config{Dir: filepath.Join(env.t.TempDir(), "tls"), Directory: "https://acme.invalid/dir", MaxPerUser: 10}, env.st, env.srv.eng, quiet)
 	env.srv.cfg.Certs = m
 	return m
 }