Scene Manager: request publishes missing on this site

- Request (globe) on casting rows and loaded assets whose latest publish
  is not on this site, and Request Missing for all of them, through the
  new wtdcmds command wtd_request_revisions. Requested publishes show
  "requested, waiting for the download".
- The version picker requests a version that is not on this site.
- A 404 explains that the session is gone or wtdcmds is too old.
- Fake server: car (vehicles) with a publish not on this site,
  wtd_request_revisions and /_test/download; 45 checks.
- Tested live: car_paul modeling.blend v001 -> Requested.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scene_manager/ipc.py

@@ -24,6 +24,7 @@ from .. import config
 # Hints added to the server's message for the statuses the user can act on.
 _STATUS_HINTS = {
     401: "the server refused the token, try again",
+    404: "the Libreflow session is gone, or its wtdcmds extension is too old: update it and restart Libreflow",
     504: "no answer within 10 s, the command may still complete in Libreflow",
 }
 # Statuses whose message replaces the server's.