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/httpx/problem.go

@@ -0,0 +1,34 @@
+// Package httpx holds small HTTP helpers shared by middleware and handlers.
+package httpx
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// Problem is an RFC 9457 problem details body.
+type Problem struct {
+	Type      string `json:"type,omitempty"`
+	Status    int    `json:"status"`
+	Title     string `json:"title"`
+	Detail    string `json:"detail,omitempty"`
+	Instance  string `json:"instance,omitempty"`
+	RequestID string `json:"requestId,omitempty"`
+}
+
+// WriteProblem writes a problem+json error response.
+func WriteProblem(w http.ResponseWriter, r *http.Request, status int, detail string) {
+	p := Problem{
+		Type:   "about:blank",
+		Title:  http.StatusText(status),
+		Status: status,
+		Detail: detail,
+	}
+	if r != nil {
+		p.RequestID = RequestID(r.Context())
+	}
+	w.Header().Set("Content-Type", "application/problem+json")
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(p) //nolint:errcheck,gosec // the client may have gone; nothing to do
+}