chore: add memo_relation (#1585)

store/db/migration/dev/LATEST__SCHEMA.sql

@@ -123,4 +123,12 @@ CREATE TABLE idp (
   type TEXT NOT NULL,
   identifier_filter TEXT NOT NULL DEFAULT '',
   config TEXT NOT NULL DEFAULT '{}'
+);
+
+-- memo_relation
+CREATE TABLE memo_relation (
+  memo_id INTEGER NOT NULL,
+  related_memo_id INTEGER NOT NULL,
+  type TEXT NOT NULL,
+  UNIQUE(memo_id, related_memo_id, type)
 );