default to main event for batch tokens

fix prev_batch token for legacy sync timeline

Signed-off-by: Jason Volk <jason@zemos.net>

src/api/client/context.rs

@@ -169,12 +169,14 @@ pub(crate) async fn get_context_route(
 		start: events_before
 			.last()
 			.map(at!(0))
+			.or(Some(base_token))
 			.as_ref()
 			.map(ToString::to_string),
 
 		end: events_after
 			.last()
 			.map(at!(0))
+			.or(Some(base_token))
 			.as_ref()
 			.map(ToString::to_string),