Wrong retention policy being used when filtering events

Fix loop var being used outside block.

Before this change, we were always using the last room_id's
retention policy for all events being filtered.

I found this with astral-sh/ruff#11769

Author: MadLittleMods

synapse/visibility.py

@@ -151,7 +151,7 @@ def allowed(event: EventBase) -> Optional[EventBase]:
             filter_send_to_client=filter_send_to_client,
             sender_ignored=event.sender in ignore_list,
             always_include_ids=always_include_ids,
-            retention_policy=retention_policies[room_id],
+            retention_policy=retention_policies[event.room_id],
             state=state_after_event,
             is_peeking=is_peeking,
             sender_erased=erased_senders.get(event.sender, False),