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/PeerTubeIntegrationTest.php

@@ -0,0 +1,136 @@
+<?php
+
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Response;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * PeerTubeIntegrationTest — regression guard for the HTTPS/scheme changes.
+ *
+ * The session-cookie hardening (RequestUtil::isSecure) changed how the app
+ * detects the request scheme. PeerTube must be unaffected: the service talks
+ * to an *absolute* instance_url stored in peertube_settings, and must never
+ * derive or rewrite that URL from $_SERVER.
+ *
+ * These tests pin the exact URLs PeerTubeService builds, so a future refactor
+ * that starts routing PeerTube through request-derived base URLs fails loudly.
+ *
+ * Uses Guzzle's MockHandler so no network is touched.
+ */
+class PeerTubeIntegrationTest extends TestCase
+{
+    private Database $db;
+    private MockHandler $mock;
+
+    protected function setUp(): void
+    {
+        $this->db = TestDatabase::getInstance();
+        $this->db->getPDO()->exec('DELETE FROM peertube_settings');
+        $this->db->getPDO()->exec('INSERT INTO peertube_settings (id) VALUES (1)');
+        // TestDatabase is a shared singleton — reset the feature flag we toggle.
+        $this->db->getPDO()
+            ->exec("DELETE FROM site_settings WHERE key = 'peertube_upload_enabled'");
+
+        $this->mock = new MockHandler();
+        $stack      = HandlerStack::create($this->mock);
+        PeerTubeService::setHttpClient(new Client([
+            'handler'        => $stack,
+            'http_errors'    => false,
+            'allow_redirects' => false,
+        ]));
+    }
+
+    protected function tearDown(): void
+    {
+        PeerTubeService::setHttpClient(null);
+        $this->db->getPDO()
+            ->exec("DELETE FROM site_settings WHERE key = 'peertube_upload_enabled'");
+    }
+
+    /** Queue an OAuth client + token handshake, then return the request index. */
+    private function queueAuth(string $instanceUrl): void
+    {
+        $this->mock->append(new Response(200, [], json_encode([
+            'client_id'     => 'cid',
+            'client_secret' => 'csec',
+        ])));
+        $this->mock->append(new Response(200, [], json_encode([
+            'access_token' => 'tok',
+        ])));
+    }
+
+    /**
+     * Settings are stored and read back verbatim — instance_url is absolute,
+     * independent of any request context.
+     */
+    public function testInstanceUrlStoredVerbatim(): void
+    {
+        PeerTubeService::updateSettings($this->db, [
+            'instance_url' => 'https://video.erg.be/',
+            'channel_name' => 'tfe@video.erg.be',
+            'privacy'      => 2,
+        ]);
+
+        $s = PeerTubeService::getSettings($this->db);
+        $this->assertSame('https://video.erg.be', $s['instance_url'], 'trailing slash trimmed');
+        $this->assertSame('tfe@video.erg.be', $s['channel_name']);
+        $this->assertSame(2, $s['privacy']);
+    }
+
+    /**
+     * probeAuth() must hit the absolute instance_url, not a request-derived
+     * host. Changing HTTPS detection must not alter these URLs.
+     */
+    public function testProbeAuthUsesAbsoluteInstanceUrl(): void
+    {
+        $s = [
+            'instance_url' => 'https://video.erg.be',
+            'username'     => 'user',
+            'password'     => 'pass',
+        ];
+
+        $this->queueAuth('https://video.erg.be');
+
+        $result = PeerTubeService::probeAuth($s);
+        $this->assertTrue($result['ok'], $result['error']);
+
+        // Last request in the handshake is the token exchange; it must target
+        // the absolute configured instance URL.
+        $this->assertSame(
+            'https://video.erg.be/api/v1/users/token',
+            (string) $this->mock->getLastRequest()->getUri()
+        );
+    }
+
+    /**
+     * An HTTP instance_url must be preserved as-is. If URL handling ever
+     * started "upgrading" to https based on request scheme, this fails.
+     */
+    public function testHttpInstanceUrlPreservedWhenConfigured(): void
+    {
+        PeerTubeService::updateSettings($this->db, [
+            'instance_url' => 'http://localhost:9000',
+            'channel_name' => 'chan',
+        ]);
+
+        $s = PeerTubeService::getSettings($this->db);
+        $this->assertSame('http://localhost:9000', $s['instance_url']);
+    }
+
+    /**
+     * isEnabled() is a pure feature-flag read and must not depend on request
+     * scheme or HTTPS detection.
+     */
+    public function testFeatureFlagIndependentOfRequestScheme(): void
+    {
+        $this->assertFalse(PeerTubeService::isEnabled($this->db));
+
+        $this->db->getPDO()
+            ->prepare("INSERT OR REPLACE INTO site_settings (key, value) VALUES ('peertube_upload_enabled', '1')")
+            ->execute();
+
+        $this->assertTrue(PeerTubeService::isEnabled($this->db));
+    }
+}