fix(security): make session-cookie hardening work under TLS termination

The passive scan flagged 3x MEDIUM 'PHPSESSID missing Secure' and an
admin-login cookie missing HttpOnly. Root cause was not the cookie code
but scheme detection: every call site read $_SERVER['HTTPS'] directly,
which is unset when TLS is terminated in front of PHP-FPM, so the Secure
flag was silently dropped.

Add RequestUtil::isSecure()/baseUrl() as the single scheme detector:
  1. $_SERVER['HTTPS'] (direct TLS)
  2. X-Forwarded-Proto — only from a loopback/private peer, so a remote
     client cannot spoof it
  3. SERVER_PORT == 443
Replace all 8 inline $_SERVER['HTTPS'] idioms (App, AdminAuth, 4x filepond
endpoints, validate-access, admin/acces, SystemController self-ping).

nginx: forward the scheme to php-fpm in all 4 fastcgi blocks
(fastcgi_param HTTPS $https + X-Forwarded-Proto $scheme) and uncomment
server_tokens off. Config validated with nginx -t (real config, alpine
container).

AdminAuth: logout() now clears the session cookie with the same
path/domain/secure/httponly/samesite it was set with, instead of the
legacy positional setcookie() that dropped SameSite.

Tests (+24, 340 total):
  - RequestUtilTest: scheme detection incl. proxy trust boundaries
  - SessionCookieTest: logout emits a hardened deletion cookie
  - PeerTubeIntegrationTest: PeerTube keeps using its absolute instance_url
    and is unaffected by scheme changes (Guzzle MockHandler)
  - NginxConfigTest: pins HTTPS forwarding, HSTS includeSubDomains,
    server_tokens off

PeerTube uses an absolute instance_url from the DB and never derives URLs
from the request, so the integration is unchanged — the regression tests
lock that invariant in.

app/public/validate-access.php

@@ -164,8 +164,9 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     $sessionToken = $db->createAccessSession($redemption['request_id'], 30);
 
     // Set HttpOnly, Secure, SameSite=Strict cookie (long-lived session)
+    require_once APP_ROOT . '/src/RequestUtil.php';
     $cookieName   = 'tfe_access_' . $thesisId;
-    $cookieSecure = !empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off';
+    $cookieSecure = RequestUtil::isSecure();
 
     setcookie($cookieName, $sessionToken, [
         'expires'  => time() + 30 * 86400,