fix(search): make closed DM search work correctly (#37)

fluxer_devops/cassandra/migrations/20260106001944_user_dm_history.cql

@@ -0,0 +1,5 @@
+CREATE TABLE IF NOT EXISTS fluxer.user_dm_history (
+    user_id bigint,
+    channel_id bigint,
+    PRIMARY KEY ((user_id), channel_id)
+);