Add Image id column to the database schema

persistence/schema.sql

@@ -59,6 +59,7 @@ create table urls (rowid integer primary key,
 );
 
 create table images (rowid integer primary key,
+    id integer unique not null check(typeof(id) = 'integer'),
     tweet_id integer not null,
     filename text not null unique,
     is_downloaded boolean default 0,