Safer defaults, read-only role, security headers and syslog TCP limits

- ALLOW_PURGE is now false by default; the UI shows a banner when there is
  no authentication.
- Read-only role: AUTH_VIEWER_USER/AUTH_VIEWER_PASS in local mode, or
  OIDC_ADMIN_GROUP in OIDC mode; changes get 403 and the admin settings
  are greyed out.
- Content-Security-Policy (inline scripts allowed by hash) and other
  security headers; cross-site changes are refused.
- Syslog TCP: at most SYSLOG_TCP_MAX_CONNS connections, closed after
  SYSLOG_TCP_IDLE of silence; HTTP idle timeout.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

web/index.html

@@ -97,6 +97,12 @@
 
   <section id="histo" class="histo" data-i18n-aria="histoAria" hidden></section>
 
+  <div id="authWarn" class="auth-warn" role="status" hidden>
+    <span data-i18n="authOff"></span>
+    <button id="authWarnClose" class="icon-btn" type="button" data-i18n-aria="close">
+      <svg viewBox="0 0 24 24"><path d="M6 6l12 12M18 6 6 18"/></svg>
+    </button>
+  </div>
   <div id="error" class="error-banner" hidden></div>
   <button id="newPill" class="pill" type="button" hidden></button>