feat(surf): print the serve URL and admin status on startup

- Add surf.Router.AdminPrefix (additive): the admin mount path and true
  when BuildRouter mounted the cabana admin, false otherwise
- serve binds with net.Listen before printing, so a busy port fails
  without a misleading URL, then prints the listen address with a
  browser URL and an admin status line: the admin URL, a warning naming
  summer admin:create when backend_users is empty or missing, or
  admin: off with the steps to add an admin controller
- The administrator count is bounded by a 2 s timeout and never fails boot
- route:list ends with the same admin status line
- surf and cabana READMEs and the console and routing docs describe the output

modules/surf/admin_prefix_test.go

@@ -1,6 +1,7 @@
 package surf
 
 import (
+	"bytes"
 	"io/fs"
 	"net/http"
 	"os"
@@ -10,6 +11,7 @@ import (
 	"testing/fstest"
 
 	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/bonfire"
 	"git.golem15.com/golem15/summercms/modules/compass"
 	"git.golem15.com/golem15/summercms/modules/pact"
 	"git.golem15.com/golem15/summercms/modules/party"
@@ -177,3 +179,57 @@ func (p prefixRoutePlugin) Routes(r pact.Router) error {
 	})
 	return nil
 }
+
+// TestRouterAdminPrefix reports the admin mount path BuildRouter used.
+func TestRouterAdminPrefix(t *testing.T) {
+	for _, tc := range []struct {
+		name    string
+		app     *backpack.App
+		plugins []party.Plugin
+		prefix  string
+		mounted bool
+	}{
+		{"configured prefix", adminPrefixApp(t), []party.Plugin{adminPrefixPlugin{}}, "/acme-admin", true},
+		{"default prefix", adminPrefixAppWith(t, ""), []party.Plugin{adminPrefixPlugin{}}, "/backend", true},
+		{"no admin controllers", adminPrefixApp(t), []party.Plugin{prefixRoutePlugin{id: "acme.site", method: http.MethodGet, path: "/hello"}}, "", false},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			r, err := BuildRouter(tc.app, tc.plugins)
+			if err != nil {
+				t.Fatal(err)
+			}
+			prefix, mounted := r.AdminPrefix()
+			if prefix != tc.prefix || mounted != tc.mounted {
+				t.Fatalf("AdminPrefix() = (%q, %v), want (%q, %v)", prefix, mounted, tc.prefix, tc.mounted)
+			}
+		})
+	}
+	if prefix, mounted := (*Router)(nil).AdminPrefix(); prefix != "" || mounted {
+		t.Fatalf("nil router AdminPrefix() = (%q, %v)", prefix, mounted)
+	}
+}
+
+// TestRouteListAdminStatus ends the route table with the admin status line.
+func TestRouteListAdminStatus(t *testing.T) {
+	for _, tc := range []struct {
+		name    string
+		app     *backpack.App
+		plugins []party.Plugin
+		want    string
+	}{
+		{"mounted", adminPrefixApp(t), []party.Plugin{adminPrefixPlugin{}}, "admin: mounted at /acme-admin"},
+		{"off", adminPrefixApp(t), []party.Plugin{prefixRoutePlugin{id: "acme.site", method: http.MethodGet, path: "/hello"}}, "admin: off — no plugin registers an admin controller"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			out := bonfire.NewOutput(strings.NewReader(""), &buf, &buf)
+			if err := RouteListCommand(tc.app, tc.plugins).Run(t.Context(), nil, out); err != nil {
+				t.Fatal(err)
+			}
+			got := strings.TrimRight(buf.String(), "\n")
+			if !strings.HasSuffix(got, tc.want) {
+				t.Fatalf("route:list output does not end with %q:\n%s", tc.want, got)
+			}
+		})
+	}
+}