Add an httpreq endpoint for lego and Traefik

jarld can now answer lego's httpreq DNS provider
(POST /api/v1/acme/httpreq/present and /cleanup), so lego and Traefik
publish DNS-01 challenges with only environment variables, without the
jarl CLI. lego's username and API token arrive as HTTP Basic auth,
accepted on these routes only.

The endpoint is off by default (acme.httpreq.enabled) and HTTPS-only by
default: jarld's own TLS, or a trusted proxy that says so with
X-Forwarded-Proto or Forwarded (the last value counts). Plain HTTP can be
allowed from loopback only or everywhere. GET /api/v1/acme/httpreq shows
how jarld sees a request, to check a proxy setup.

jarl acme and the endpoint share their code in internal/acme. Documented
in docs/acme.md, configuration.md, api.md and permissions.md.

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

internal/config/config_test.go

@@ -18,6 +18,22 @@ func TestExampleParses(t *testing.T) {
 	if c.BIND.ZoneDir != d.BIND.ZoneDir || time.Duration(c.Session.IdleTimeout) != 2*time.Hour || c.Listen[0] != "127.0.0.1:8053" {
 		t.Errorf("example differs from defaults: %+v", c)
 	}
+	if c.ACME != d.ACME || c.ACME.HTTPReq.Enabled || !c.ACME.HTTPReq.RequireHTTPS {
+		t.Errorf("acme = %+v, want the defaults %+v", c.ACME, d.ACME)
+	}
+}
+
+func TestHTTPReqDefaults(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "jarl.yaml")
+	// Enabling the endpoint keeps require_https on unless it is turned off.
+	os.WriteFile(path, []byte("acme: {httpreq: {enabled: true}}\n"), 0o644)
+	c, err := Load(path, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if h := c.ACME.HTTPReq; !h.Enabled || !h.RequireHTTPS || h.AllowLoopbackHTTP {
+		t.Errorf("httpreq = %+v", h)
+	}
 }
 
 func TestLoad(t *testing.T) {