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_ratelimitrule.go

@@ -0,0 +1,29 @@
+package config
+
+import "reflect"
+
+// JSCRateLimitRule configures one rate limit. The type is used under several
+// keys so it has no JSCK* constant and its Flatten keys are not prefixed.
+type JSCRateLimitRule struct {
+	Enabled   bool `cfg:"enabled"             default:"false"`
+	PerSecond int  `cfg:"per_second" default:"50"`
+	Burst     int  `cfg:"burst"               default:"100"`
+}
+
+// ---------- Section methods (see Section) ----------
+
+func (c JSCRateLimitRule) Map() map[string]any { return mapOf(c) }
+
+func (c JSCRateLimitRule) Flatten() map[string]any { return flattenOf("", c) }
+
+func (c JSCRateLimitRule) TypeOf(key string) (reflect.Kind, error) {
+	return typeOf(c, key)
+}
+
+func (c JSCRateLimitRule) ValueOf(key string) (any, error) {
+	return valueOf(c, key)
+}
+
+func (c *JSCRateLimitRule) SetValue(key string, value any) error {
+	return setValue(c, key, value)
+}