Adds the new shows admin screen and drives schedule from it

backend/app/api/shows.py

@@ -89,6 +89,7 @@ async def update_show(
         # Replace all schedule slots
         for sched in show.schedules:
             await session.delete(sched)
+        await session.flush()  # execute deletes before inserts to avoid UNIQUE conflict
         for slot in payload.schedules:
             session.add(ShowSchedule(show_id=show.id, **slot.model_dump()))