Keep working out the home page cards after startup (closes #211)

Server.Start ran the loop that works out the home page's cards on the
context it was given. fx ends that context once start-up is done, so the
cards were worked out once and never again. The loop now runs on a
context that only Stop ends, as the other background parts do. Stop
waits for the loop to return, until its own context ends, before it
stops the HTTP server.

A new test starts a server in a synctest bubble and ends its context at
once. On the bubble's fake clock, the cards must be worked out again a
minute later, and not after Stop.

Model: opus-5-5

internal/server/server.go

@@ -82,6 +82,12 @@ type Server struct {
 	// cards is the copy of the home page's cards that the pages serve, nil
 	// until the server first works them out
 	cards atomic.Pointer[homeCards]
+
+	// stopCards ends the goroutine that works out the cards, and
+	// cardsStopped is closed once it has returned. Both are nil until Start
+	// starts the goroutine.
+	stopCards    context.CancelFunc
+	cardsStopped chan struct{}
 }
 
 // New creates a new HTTP server that reads routes from table, the routing
@@ -111,7 +117,8 @@ func New(
 // Start listens on the configured port and serves HTTP in the background.
 // It returns an error if it cannot listen, for example because the port is
 // already in use. ctx bounds opening the port, not serving. Once listening,
-// the server works out the home page's cards every minute until ctx ends.
+// the server works out the home page's cards every minute until Stop: the end
+// of ctx does not stop it.
 func (s *Server) Start(ctx context.Context) error {
 	const (
 		network           = "tcp"
@@ -148,16 +155,22 @@ func (s *Server) Start(ctx context.Context) error {
 		}
 	}()
 
+	ctx, s.stopCards = context.WithCancel(context.WithoutCancel(ctx))
+	s.cardsStopped = make(chan struct{})
+
 	go func() {
+		defer close(s.cardsStopped)
+
 		s.keepHomeCardsFresh(ctx, homeCardsRefreshInterval)
 	}()
 
 	return nil
 }
 
-// Stop stops the HTTP server. It waits, until ctx ends, for the requests in
-// progress to finish, and then closes the connections of those still running
-// and returns ctx's error.
+// Stop stops the goroutine that works out the home page's cards, and waits
+// until it has returned or ctx ends. It then stops the HTTP server. It waits,
+// until ctx ends, for the requests in progress to finish, and then closes the
+// connections of those still running and returns ctx's error.
 func (s *Server) Stop(ctx context.Context) error {
 	if s.srv == nil {
 		return nil
@@ -165,6 +178,16 @@ func (s *Server) Stop(ctx context.Context) error {
 
 	s.logger.Info("Stopping HTTP server")
 
+	// Start returns before it starts the goroutine when it cannot listen.
+	if s.stopCards != nil {
+		s.stopCards()
+
+		select {
+		case <-s.cardsStopped:
+		case <-ctx.Done():
+		}
+	}
+
 	err := s.srv.Shutdown(ctx)
 	if err != nil {
 		return errors.Join(err, s.srv.Close())