Drop unused thread_id from questions

internal/store/migrations/075-drop-inbox-thread.sql

@@ -0,0 +1,4 @@
+ALTER TABLE inbox_items DROP COLUMN thread_id;
+
+UPDATE events SET payload = json_remove(payload, '$.thread_id')
+WHERE kind IN ('inbox.created', 'inbox.updated') AND json_type(payload, '$.thread_id') IS NOT NULL;