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.

tests/phpunit/NginxConfigTest.php

@@ -0,0 +1,85 @@
+<?php
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * NginxConfigTest — regression guard for the passive-scan hardening.
+ *
+ * The scan found live responses where:
+ *   - HSTS lacked includeSubDomains (config drift),
+ *   - PHP did not know the request was HTTPS (Secure cookie flag dropped),
+ *   - the nginx version was disclosed via the Server header.
+ *
+ * The PHP-side fixes only work if nginx forwards the scheme to PHP-FPM and
+ * hides its version. These assertions pin the config so a later edit can't
+ * silently undo them.
+ *
+ * Full syntax validation still runs server-side via `nginx -t` in
+ * scripts/deploy-server.sh.
+ */
+class NginxConfigTest extends TestCase
+{
+    private string $conf;
+
+    protected function setUp(): void
+    {
+        $path = APP_ROOT . '/../nginx/xamxam.conf';
+        $this->assertFileExists($path);
+        $this->conf = (string) file_get_contents($path);
+    }
+
+    public function testServerTokensDisabled(): void
+    {
+        $this->assertMatchesRegularExpression(
+            '/^\s*server_tokens\s+off;/m',
+            $this->conf,
+            'server_tokens must be active (not commented) to hide the nginx version'
+        );
+    }
+
+    public function testHstsIncludesSubdomainsAndPreload(): void
+    {
+        $this->assertMatchesRegularExpression(
+            '/add_header\s+Strict-Transport-Security\s+"max-age=\d+;\s*includeSubDomains;\s*preload;"/',
+            $this->conf,
+            'HSTS must include includeSubDomains; preload'
+        );
+    }
+
+    /**
+     * Every php-fpm location must forward HTTPS + the original scheme, or
+     * RequestUtil falls back to insecure and cookies lose the Secure flag.
+     */
+    public function testEveryPhpLocationForwardsHttpsSignal(): void
+    {
+        $phpValueCount  = preg_match_all('/fastcgi_param\s+PHP_VALUE\s+"upload_max_filesize/m', $this->conf);
+        $httpsCount     = preg_match_all('/fastcgi_param\s+HTTPS\s+\$https;/m', $this->conf);
+        $protoCount     = preg_match_all('/fastcgi_param\s+HTTP_X_FORWARDED_PROTO\s+\$scheme;/m', $this->conf);
+
+        $this->assertGreaterThan(0, $phpValueCount, 'expected php-fpm locations');
+        $this->assertSame(
+            $phpValueCount,
+            $httpsCount,
+            "every php-fpm block must set fastcgi_param HTTPS (\$phpValue=$phpValueCount, https=$httpsCount)"
+        );
+        $this->assertSame(
+            $phpValueCount,
+            $protoCount,
+            "every php-fpm block must set X-Forwarded-Proto (\$phpValue=$phpValueCount, proto=$protoCount)"
+        );
+    }
+
+    /**
+     * The admin location must keep its CSP and noindex header, and must not
+     * inherit a broken frame-ancestors from the parent.
+     */
+    public function testAdminLocationKeepsSecurityHeaders(): void
+    {
+        $this->assertMatchesRegularExpression(
+            '#location\s+\^~\s+/admin/\s*\{.*?add_header\s+Content-Security-Policy#s',
+            $this->conf,
+            'admin location must define its own CSP'
+        );
+        $this->assertStringContainsString('X-Robots-Tag "noindex, nofollow"', $this->conf);
+    }
+}