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>

src/routes/events/[event_id]/(launcher)/launcher/+layout.svelte

@@ -197,9 +197,7 @@ $effect(() => {
         untrack(() => {
             $events_slct.event_device_id =
                 native_dev.event_device_id ||
-                native_dev.id ||
-                native_dev.event_device_id_random ||
-                native_dev.id_random;
+                native_dev.id;
         });
     }
 });