Implement TFE file access restriction feature (complete)

Requirements:
- parametres.php toggle: 'restricted_files_enabled' enables/disables the feature
- Public TFE page: when enabled + access_type=Interne, hides files, shows French
  restriction message + access request form (metadata/synopsis still visible)
- ERG emails (@erg.school / @erg.be): auto-approve, send 24h access link immediately
- External emails: show justification textarea, create pending request, notify admin
- Admin panel /admin/file-access.php: approve/reject requests with optional notes,
  sends access email on approval (linked from admin nav with pending count badge)

Security:
- One-time 24h email tokens (used_at + is_valid=0 on first click)
- Token redeemed via POST /validate-access (GET shows confirmation page only)
- Long-lived 30-day browser session in file_access_sessions table
- Cookie: HttpOnly + Secure + SameSite=Strict
- CSRF on all mutations, rate limiting on request submission
- Audit trail: IP, UA, event, timestamp in file_access_audit

Bug fixes:
- admin/file-access.php: $vars never extract()ed → page was blank
- Template had self-contained head/footer includes (double-include)
- Admin approval URL used $requestId instead of $request['thesis_id']
- App::boot() now starts session so CSRF token works on public pages
- Dispatcher routes /validate-access and /request-access through front controller

app/src/App.php

@@ -22,6 +22,9 @@ class App
             require_once APP_ROOT . '/src/Database.php';
             self::$booted = true;
         }
+        if (session_status() === PHP_SESSION_NONE) {
+            session_start();
+        }
         self::ensureCsrf();
         return Database::getInstance();
     }