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/auth/auth_test.go

@@ -134,3 +134,23 @@ func TestRoleTemplatesValid(t *testing.T) {
 		}
 	}
 }
+
+func TestCertificateGrants(t *testing.T) {
+	g, err := ParseGrant("certificates:write names=*.adbraun.lol")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !GrantAllows(g, Action{Resource: ResCerts, Write: true, Name: "mail.adbraun.lol"}) || GrantAllows(g, Action{Resource: ResCerts, Write: true, Name: "mail.example.org"}) {
+		t.Error("names filter")
+	}
+	if _, err := ParseGrant("certificates:read types=TXT"); err == nil {
+		t.Error("types on certificates accepted")
+	}
+	if _, err := ParseGrant("certificates:read zones=adbraun.lol"); err == nil {
+		t.Error("zones on certificates accepted")
+	}
+	// "*" never covers certificates: they hand out private keys.
+	if GrantAllows(model.Grant{Resource: ResAll, Access: Write}, Action{Resource: ResCerts, Name: "mail.adbraun.lol"}) {
+		t.Error("* covers certificates")
+	}
+}