feat(11.1-04): add the Backend section, the remaining Services pages and the concept map links

- docs/backend: admin controllers, forms, lists and filters, relation
  manager, users and permissions, settings, partials and widgets, admin SPA
- docs/services: storage, outbound HTTP, realtime, Web Push, search, parity
  testing and the Frontend and AJAX (not provided) page
- Examples for cabana (with testdata/docs YAML), fetchguard, lighthouse and
  its centrifugo driver, flare, beachcomber and typesense, tide; lighthouse
  and beachcomber TestDocs* regions run on their Postgres harnesses
- concept map rows link their guide pages and the not-provided rows the
  Frontend and AJAX page; index lists Backend, Database and Services
- TestDocsRequiredPages asserts the D-08 section order

modules/lighthouse/centrifugo/example_test.go

@@ -0,0 +1,102 @@
+package centrifugo_test
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/compass"
+	"git.golem15.com/golem15/summercms/modules/lighthouse"
+	"git.golem15.com/golem15/summercms/modules/lighthouse/centrifugo"
+	"git.golem15.com/golem15/summercms/modules/surf"
+)
+
+// newApp returns an app whose config selects a realtime driver.
+func newApp(settings map[string]any) (*backpack.App, error) {
+	cfg, err := compass.Open(compass.Options{Dir: "config", Env: "development", Environ: []string{}})
+	if err != nil {
+		return nil, err
+	}
+	for k, v := range settings {
+		if err := cfg.Set(k, v); err != nil {
+			return nil, err
+		}
+	}
+	return backpack.New(cfg), nil
+}
+
+func ExampleProxyHandler() {
+	svc, err := lighthouse.From(backpack.New(nil))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	// Members of blog 7 may subscribe to its channels.
+	err = svc.Registry().Register("blog", lighthouse.AuthorizerFunc(
+		func(ctx context.Context, userID uint, channel string) lighthouse.Result {
+			if userID == 42 && lighthouse.ChannelID(channel) == 7 {
+				return lighthouse.Allowed(nil)
+			}
+			return lighthouse.Denied("not a member of the blog")
+		}))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	// realtime.centrifugo.proxy_secret; set it through the environment.
+	proxy := centrifugo.ProxyHandler(svc, centrifugo.Config{ProxySecret: "test-only-proxy-secret"})
+
+	// What Centrifugo posts to the subscribe proxy.
+	subscribe := func(secret, user, channel string) {
+		body := fmt.Sprintf(`{"client":"c1","user":%q,"channel":%q}`, user, channel)
+		req := httptest.NewRequest(http.MethodPost, "/api/realtime/subscribe", strings.NewReader(body))
+		req.Header.Set("X-Centrifugo-Secret", secret)
+		rec := httptest.NewRecorder()
+		proxy.ServeHTTP(rec, req)
+		fmt.Println(rec.Code, strings.TrimSpace(rec.Body.String()))
+	}
+	subscribe("test-only-proxy-secret", "42", "blog:7")
+	subscribe("test-only-proxy-secret", "5", "blog:7")
+	subscribe("wrong-secret", "42", "blog:7")
+	// Output:
+	// 200 {"result":{"info":[]}}
+	// 200 {"error":{"code":403,"message":"Access denied"}}
+	// 200 {"error":{"code":403,"message":"Access denied"}}
+}
+
+func ExampleDriver_Routes() {
+	app, err := newApp(map[string]any{"realtime.driver": "centrifugo"})
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	svc, err := lighthouse.From(app)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	// In a plugin's Routes method, r is the router the plugin receives.
+	r := surf.New(nil)
+	err = lighthouse.Mount(r, svc.Driver(), lighthouse.Surfaces{
+		UserAuth:   surf.Use("acme.auth"),
+		Middleware: surf.Use("throttle:60,1"),
+	})
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	for _, rt := range r.Routes() {
+		fmt.Println(rt.Method, rt.Pattern, rt.Middleware, "raw:", rt.Raw)
+	}
+
+	// A user route without a guard is refused.
+	err = lighthouse.Mount(surf.New(nil), svc.Driver(), lighthouse.Surfaces{})
+	fmt.Println(err != nil)
+	// Output:
+	// GET /api/realtime/token [acme.auth throttle:60,1] raw: false
+	// POST /api/realtime/subscribe [throttle:60,1] raw: true
+	// true
+}