feat: dual upload system — direct file storage + PeerTube API integration

Adds a parallel PeerTube upload system behind a feature flag (disabled by default
until upload quota is granted). When disabled, the existing direct file upload
path works unchanged.

Files:
- src/PeerTubeService.php — credential storage (encrypted), OAuth2 token
  retrieval, multipart upload to /api/v1/videos/upload
- migrations/021_peertube_settings.sql — peertube_settings singleton table
  + peertube_upload_enabled site_setting (default 0)
- admin/actions/settings.php — peertube section handler
- admin/parametres.php / templates/admin/parametres.php — PeerTube UI section
- partage/fichiers-fragment.php — shows file inputs when enabled, TODO notice otherwise
- ThesisCreateController / ThesisEditController — handlePeerTubeUpload()
- tfe.php — PeerTube iframe embed detection
- AdminLogger — logPeerTubeUpdate()

app/src/AdminLogger.php

@@ -224,6 +224,12 @@ class AdminLogger
         $this->write('settings', 'smtp_update', 'success', ['connection_ok' => $connectionOk]);
     }
 
+    /** Parametres: PeerTube settings update */
+    public function logPeerTubeUpdate(bool $enabled): void
+    {
+        $this->write('settings', 'peertube_update', 'success', ['enabled' => $enabled]);
+    }
+
     /** Parametres: SMTP test */
     public function logSmtpTest(string $toEmail, bool $success, string $error = ''): void
     {