refactor: Promote handling unsigned data out of timeline

Also fixes:
- Transaction IDs leaking in event route
- Age not being set for event relations or threads
- Both of the above for search results

Notes down concern with relations table

src/api/client/relations.rs

@@ -172,6 +172,10 @@ async fn paginate_relations_with_filter(
 	})
 }
 
+// TODO: Can we move the visibility filter lower down, to avoid checking events
+// that won't be sent? At the moment this also results in getting events that
+// appear to have no relation because intermediaries are not visible to the
+// user.
 async fn visibility_filter(
 	services: &Services,
 	sender_user: &UserId,