Subquery in `FaclityViewset` `last_synced` annotation can return multiple results
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### Observed behavior

This subquery from the `TransferSession` table can return multiple results, which causes the `/api/auth/facility` endpoint to  break

https://github.com/learningequality/kolibri/blob/release-v0.14.x/kolibri/core/auth/api.py#L359

### Expected behavior

The `/api/auth/facility` endpoint doesn't break because of this query. The subquery should be limited to a returning a single value.

### User-facing consequences
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### Context
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diff --git a/kolibri/core/auth/api.py b/kolibri/core/auth/api.py
index 695786901b..5c36a4f269 100644
--- a/kolibri/core/auth/api.py
+++ b/kolibri/core/auth/api.py
@@ -366,7 +366,7 @@ class FacilityViewSet(ValuesViewset):
                         )
                     )
                     .order_by("-last_activity_timestamp")
-                    .values("last_activity_timestamp")
+                    .values("last_activity_timestamp")[:1]
                 )
             )
         )
['kolibri/core/auth/api.py']