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/users.go

@@ -0,0 +1,71 @@
+package lighthouse
+
+import (
+	"context"
+
+	"git.golem15.com/golem15/summercms/modules/bouncer"
+)
+
+// User is the identity a realtime driver needs: the id and display name.
+type User struct {
+	ID   uint
+	Name *string
+}
+
+// UserLookup loads a user by id. found is false when no such user exists.
+type UserLookup func(ctx context.Context, id uint) (user User, found bool, err error)
+
+// SetUserLookup installs the application's user lookup.
+func (s *Service) SetUserLookup(fn UserLookup) {
+	if s == nil {
+		return
+	}
+	s.mu.Lock()
+	s.lookup = fn
+	s.mu.Unlock()
+}
+
+// User loads user id through the installed lookup. Without a lookup it
+// returns User{ID: id} with a nil Name and found true.
+func (s *Service) User(ctx context.Context, id uint) (User, bool, error) {
+	if s == nil {
+		return User{ID: id}, true, nil
+	}
+	s.mu.RLock()
+	fn := s.lookup
+	s.mu.RUnlock()
+	if fn == nil {
+		return User{ID: id}, true, nil
+	}
+	return fn(ctx, id)
+}
+
+// Actor is who caused a broadcast. It marshals as {"user_id":…,"name":…}.
+type Actor struct {
+	UserID *uint   `json:"user_id"`
+	Name   *string `json:"name"`
+}
+
+// SystemActor is the actor of writes made without a signed-in user:
+// {"user_id":null,"name":"System"}.
+func SystemActor() Actor {
+	name := "System"
+	return Actor{Name: &name}
+}
+
+// Actor returns the actor of the request in ctx. No principal, or a
+// backend-admin principal, gives SystemActor: the WinterCMS frontend user is
+// empty in backend requests. Otherwise it is the principal's id and the
+// looked-up name (nil when the lookup fails or finds nothing).
+func (s *Service) Actor(ctx context.Context) Actor {
+	p, ok := bouncer.User(ctx)
+	if !ok || p == nil || p.Backend {
+		return SystemActor()
+	}
+	id := p.ID
+	a := Actor{UserID: &id}
+	if u, found, err := s.User(ctx, id); err == nil && found {
+		a.Name = u.Name
+	}
+	return a
+}