feat(11-03): add the lighthouse realtime package and its Centrifugo driver

- lighthouse: Service/From with realtime.driver selection, RegisterDriver
  registry, null/log/memory drivers, Route/Surface/Mount, users and actors
- centrifugo: HTTP API client (apikey header, 2xx success, no request
  without a key), five-generator HS256 TokenIssuer, TokenHandler with the
  WinterCMS 401/503 bodies
- module README and root modules table row

modules/lighthouse/centrifugo/driver.go

@@ -0,0 +1,77 @@
+package centrifugo
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/lighthouse"
+)
+
+// DriverName is the realtime.driver value of this driver.
+const DriverName = "centrifugo"
+
+func init() {
+	lighthouse.RegisterDriver(DriverName, func(app *backpack.App, svc *lighthouse.Service) (lighthouse.Driver, error) {
+		var cfg Config
+		if app != nil {
+			cfg = LoadConfig(app.Config)
+		} else {
+			cfg = LoadConfig(nil)
+		}
+		return NewDriver(svc, cfg, nil), nil
+	})
+}
+
+// Driver is the Centrifugo lighthouse.Driver: it publishes through Client
+// and declares the token route (UserAuth) and the subscribe proxy route
+// (ServerToServer).
+type Driver struct {
+	svc    *lighthouse.Service
+	cfg    Config
+	client *Client
+	issuer *TokenIssuer
+}
+
+// NewDriver builds the driver for svc from cfg. hc may be nil (see
+// NewClient).
+func NewDriver(svc *lighthouse.Service, cfg Config, hc *http.Client) *Driver {
+	c := NewClient(cfg, hc)
+	c.log = svc.Logger()
+	return &Driver{
+		svc:    svc,
+		cfg:    cfg,
+		client: c,
+		issuer: NewTokenIssuer(cfg.TokenSecret, cfg.TokenTTL),
+	}
+}
+
+// Name returns "centrifugo".
+func (d *Driver) Name() string { return DriverName }
+
+// Config returns the driver's configuration.
+func (d *Driver) Config() Config { return d.cfg }
+
+// Client returns the HTTP API client.
+func (d *Driver) Client() *Client { return d.client }
+
+// Issuer returns the token issuer.
+func (d *Driver) Issuer() *TokenIssuer { return d.issuer }
+
+// Publish sends event to one channel through Client.Publish.
+func (d *Driver) Publish(ctx context.Context, channel, event string, payload json.RawMessage) error {
+	return d.client.Publish(ctx, channel, event, payload)
+}
+
+// Broadcast sends event to several channels through Client.Broadcast.
+func (d *Driver) Broadcast(ctx context.Context, channels []string, event string, payload json.RawMessage) error {
+	return d.client.Broadcast(ctx, channels, event, payload)
+}
+
+// Routes returns GET token_path (UserAuth, TokenHandler).
+func (d *Driver) Routes() []lighthouse.Route {
+	return []lighthouse.Route{
+		{Name: "token", Method: http.MethodGet, Path: d.cfg.TokenPath, Surface: lighthouse.UserAuth, Handler: TokenHandler(d.svc, d.issuer)},
+	}
+}