Signage admin - import legacy Science Firebase export

Requires permission: ScienceDisplayManagers

One-off migration. POST the raw Firebase export JSON (either the whole document with a top-level 'Data' object, or the 'Data' object itself). Walks Data/yy/MM/dd, reconstructs 20yy-MM-dd, and inserts any room/announcement not already present (idempotent - safe to re-run). Requires the ScienceDisplayManagers permission.