Added the HTTP server and serve command, with proxy, CSRF, TLS and security header config; wsserve is now ws serve, alongside a new ws dial

The serve command runs internal/server: the API under /api (health, and
the CSRF token endpoint) and the WebSocket hub under /ws, over HTTP or TLS,
with an optional HTTP to HTTPS redirect, the PROXY protocol from trusted
sources and a graceful shutdown that closes WebSockets first. Its middleware
covers the client IP behind trusted proxies, request IDs, request logging,
panic recovery, global and API rate limits, Host and body size checks,
security response headers, compression, CSRF protection (cross-origin
checks and signed double-submit tokens) and JSON-only API bodies. Middleware
that logs takes a WithLogger option.

New server config sections: proxy, csrf, tls, headers and frontend (not
used yet), plus max_header_bytes, max_body_bytes, global and API rate limits
and HTTP read, write and idle timeouts. Every setting documents what 0 or
empty means, and internal/config/README.md is reorganised by section.
server.timeouts.read_header must be greater than 0, and a header or idle
timeout of 0 means no limit rather than the read timeout.

WebSocket upgrades are protected by the Origin check: the server's own
origin, or one of csrf.cross_origin_protection.trusted_origins, which the
hub now takes with WithTrustedOrigins.

wsserve becomes "ws serve". "ws dial" opens a WebSocket on /ws to test it:
--ping sends a ping and waits for the pong, --message sends messages and
prints the replies, --origin tests the Origin check, and with neither
--ping nor --message each line of stdin is sent until Ctrl+C.

internal/config/conf_server_timeouts.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"errors"
 	"reflect"
 	"time"
 )
@@ -8,11 +9,19 @@ import (
 // JSCKServerTimeouts is the server timeouts section's configuration key.
 const JSCKServerTimeouts string = "timeouts"
 
-// JSCServerTimeouts configures the server's timeouts. ShutdownTimeout is how
-// long shutdown waits for clients to close; 0 or less means it waits until
-// they have.
+// JSCServerTimeouts configures the server's HTTP timeouts, as those of
+// http.Server: ReadHeaderTimeout for reading a request's headers,
+// ReadTimeout for the whole request, WriteTimeout for writing the response
+// and IdleTimeout for waiting for the next request on a keep-alive
+// connection. 0 or less means no limit, except that ReadHeaderTimeout must be
+// greater than 0 (see validate). ShutdownTimeout is how long shutdown waits
+// for clients to close; 0 or less means it waits until they have.
 type JSCServerTimeouts struct {
-	ShutdownTimeout time.Duration `cfg:"shutdown" default:"15s"`
+	ReadHeaderTimeout time.Duration `cfg:"read_header" default:"5s"`
+	ReadTimeout       time.Duration `cfg:"read"        default:"15s"`
+	WriteTimeout      time.Duration `cfg:"write"       default:"30s"`
+	IdleTimeout       time.Duration `cfg:"idle"        default:"2m"`
+	ShutdownTimeout   time.Duration `cfg:"shutdown"    default:"15s"`
 }
 
 // ---------- Section methods (see Section) ----------
@@ -39,3 +48,17 @@ func (c JSCServerTimeouts) ValueOf(key string) (any, error) {
 func (c *JSCServerTimeouts) SetValue(key string, value any) error {
 	return setValue(c, key, value)
 }
+
+// ---------- Section specific methods ----------
+
+// validate checks values before they are stored (see validator).
+// read_header must be greater than 0: without it a client could hold a
+// connection open for ever by sending its headers slowly (slowloris).
+func (c *JSCServerTimeouts) validate(key string, value any) error {
+	if key == "read_header" && value.(time.Duration) <= 0 {
+		return errors.New(
+			"config: read_header must be greater than 0, as a client could otherwise hold a connection open for ever",
+		)
+	}
+	return nil
+}