Refine report schema

script/sql/create/22.report.sql

@@ -1,7 +1,5 @@
 -- [REPORT]
 
--- TODO: constraint
-
 create table report
 (
   key   text not null
@@ -18,3 +16,10 @@ insert into report (key, value) values
 , ('NODES', 'NONE')
 , ('LINKS', 'NONE')
 ;
+
+create table variable_report
+(
+  content text primary key
+);
+
+insert into variable_report (content) values ('');