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/modify_test.go

@@ -181,9 +181,10 @@ func TestParseValue(t *testing.T) {
 		{reflect.Int8, "7", int64(7)},
 		{reflect.Uint16, "65535", uint64(65535)},
 		{reflect.Float32, "1.5", 1.5},
+		{reflect.Slice, " a, b ,, c ", []string{"a", "b", "c"}},
 	} {
 		got, err := parseValue(tt.kind, tt.value)
-		if err != nil || got != tt.want {
+		if err != nil || !reflect.DeepEqual(got, tt.want) {
 			t.Errorf("parseValue(%v, %q) = %#v, %v; want %#v", tt.kind, tt.value, got, err, tt.want)
 		}
 	}
@@ -202,8 +203,8 @@ func TestParseValue(t *testing.T) {
 		}
 	}
 
-	if _, err := parseValue(reflect.Slice, "a,b"); !errors.Is(err, errors.ErrUnsupported) {
-		t.Errorf("parseValue(slice) err = %v; want ErrUnsupported", err)
+	if _, err := parseValue(reflect.Map, "a"); !errors.Is(err, errors.ErrUnsupported) {
+		t.Errorf("parseValue(map) err = %v; want ErrUnsupported", err)
 	}
 }