Add backfill for topic_schedule.latest_topic_id

I doubt the absence of this would have ever been noticed or that adding
this will ever matter for anyone, but I might as well do it properly!

tildes/alembic/versions/468cf81f4a6b_topic_schedule_add_latest_topic_id.py

@@ -28,6 +28,16 @@ def upgrade():
         ["topic_id"],
     )
 
+    op.execute(
+        """
+        update topic_schedule set latest_topic_id = (
+            select topic_id from topics
+            where schedule_id = topic_schedule.schedule_id
+            order by created_time desc limit 1
+        )
+    """
+    )
+
     op.execute(
         """
         create or replace function update_topic_schedule_latest_topic_id() returns trigger as $$