Added websocket hub for handling websocket clients, with a wsserve command

which runs a plain TCP server for receiving websocket hub clients, with
server config, CSRF and rate limit middleware.

- internal/websocket: Hub pushing messages to WebSocket clients (ServeHTTP,
  example.com/ws upgrade with optional CSRF check) and plain TCP clients (ServeTCP,
  newline-delimited JSON envelopes); ping/pong keepalive, per-client message
  rate limit, slow consumer handling, graceful Shutdown, debug logging of
  messages sent and received
- internal/websocket/tcpserver: standalone TCP server for a hub, with accept
  back-off, a per-IP connection limit (server.websocket.connection_limit) and
  graceful shutdown
- internal/cli/wsserve: `wsserve` command running the TCP server, with
  --listen/--host/--port overrides
- internal/middleware: keyed token bucket rate limiter, real client IP from
  trusted proxies
- internal/httpx: RFC 9457 problem responses, request IDs in context
- internal/config: server section (host, port, shutdown timeout, rate limiter
  idle TTL, websocket); util.ByteSize settings (e.g. "32KiB"); lists of
  sections as single keys; empty lists stored as nil; isExplicitlySet for
  printLoadedConfig; JobSearchConfigSource renamed to JSCSourcesSource;
  TestSections checks every section's methods
- internal/util: ByteSize, parsed and formatted with go-humanize

internal/config/conf_sections_test.go

@@ -50,10 +50,10 @@ func TestDefaults(t *testing.T) {
 	if conf.Sources.FTSMock.APIRoot != "" {
 		t.Errorf("FTSMock.APIRoot = %q; want empty", conf.Sources.FTSMock.APIRoot)
 	}
-	for name, src := range map[string]JobSearchConfigSource{
-		"find_a_tender": conf.Sources.FindATender.JobSearchConfigSource,
-		"fts_mock":      conf.Sources.FTSMock.JobSearchConfigSource,
-		"sleeper":       conf.Sources.Sleeper.JobSearchConfigSource,
+	for name, src := range map[string]JSCSourcesSource{
+		"find_a_tender": conf.Sources.FindATender.JSCSourcesSource,
+		"fts_mock":      conf.Sources.FTSMock.JSCSourcesSource,
+		"sleeper":       conf.Sources.Sleeper.JSCSourcesSource,
 	} {
 		if src.Enabled || src.Interval != 10*time.Minute {
 			t.Errorf("%s source = %+v; want disabled, 10m", name, src)
@@ -150,7 +150,7 @@ func TestConfigLevelToLogLevel(t *testing.T) {
 }
 
 func TestSourceSection(t *testing.T) {
-	c := JobSearchConfigSource{Enabled: true, Interval: time.Minute}
+	c := JSCSourcesSource{Enabled: true, Interval: time.Minute}
 
 	if got, want := c.Keys(), []string{"enabled", "interval"}; !reflect.DeepEqual(got, want) {
 		t.Errorf("Keys = %v; want %v", got, want)
@@ -352,6 +352,7 @@ func TestSerialisedRoundTrip(t *testing.T) {
 	orig.Logging.Level = "info"
 	orig.Sources.FTSMock.Enabled = true
 	orig.Sources.FTSMock.APIRoot = "http://mock"
+	orig.Server.WebSocket.MaxMessageSize = 64 << 10
 	for _, format := range []string{"json", "yaml"} {
 		t.Run(format, func(t *testing.T) {
 			resetViper(t)