add env variable to disable direct invitation links

flake.nix

@@ -44,6 +44,7 @@
 
             UPLOAD_PATH = "./.data/uploads";
             VIDEO_PATH = "./.data/videos";
+            DIRECT_INVITATION_LINKS_ENABLED = "true";
             HOMEPAGE_TRAILER_URL = "http://localhost:4321/craftattack/video/out-final-final/embed";
 
             TEAMSPEAK_LINK = "http://example.com";