Remove _random ID references and fix hosted file download ID

- Fix download button to use hosted_file_id instead of id (which resolved
  to event_file_id via _process_generic_props, hitting the wrong endpoint)
- Fix Dexie file table query in event_file_obj_tbl_wrapper to use _id
  fields (the indexed ones) instead of _id_random variants
- Remove _random fields from properties_to_save in event, event_session
- Drop _id_random fallbacks from launcher device ID resolution and
  background sync heartbeat
- Clean up dead comments and old FA anchor in post edit component
- Update TODO__Agents.md: BGH section removed, CMSC/Axonius shows added,
  download button fix marked complete

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

aether_app_sveltekit.code-workspace

@@ -9,6 +9,7 @@
             "autofetch",
             "Axonius",
             "displayplacer",
+            "elif",
             "filelist",
             "gsettings",
             "onsave"