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_sources.go

@@ -38,32 +38,32 @@ func (c *JSCSources) SetValue(key string, value any) error {
 
 // ========== Generic source ==========
 
-// JobSearchConfigSource holds the settings every source has, and is embedded
+// JSCSourcesSource holds the settings every source has, and is embedded
 // in each source's section. Interval is how often an enabled source fetches.
-type JobSearchConfigSource struct {
+type JSCSourcesSource struct {
 	Enabled  bool          `cfg:"enabled"`
 	Interval time.Duration `cfg:"interval" default:"10m"`
 }
 
 // ---------- Section methods (see Section) ----------
 
-func (c JobSearchConfigSource) Map() map[string]any { return mapOf(c) }
+func (c JSCSourcesSource) Map() map[string]any { return mapOf(c) }
 
-func (c JobSearchConfigSource) Flatten() map[string]any {
+func (c JSCSourcesSource) Flatten() map[string]any {
 	return flattenOf("", c)
 }
 
-func (c JobSearchConfigSource) TypeOf(key string) (reflect.Kind, error) {
+func (c JSCSourcesSource) TypeOf(key string) (reflect.Kind, error) {
 	return typeOf(c, key)
 }
 
-func (c JobSearchConfigSource) ValueOf(key string) (any, error) {
+func (c JSCSourcesSource) ValueOf(key string) (any, error) {
 	return valueOf(c, key)
 }
 
-func (c *JobSearchConfigSource) SetValue(key string, value any) error {
+func (c *JSCSourcesSource) SetValue(key string, value any) error {
 	return setValue(c, key, value)
 }
 
 // Keys returns the keys directly in the section.
-func (c *JobSearchConfigSource) Keys() []string { return keysOf(c) }
+func (c *JSCSourcesSource) Keys() []string { return keysOf(c) }