Add index to sets.created

Most queries will be filtering on created date
(showing todays sets, todays plans) so this
should speed up our queries.

db.ts

@@ -117,6 +117,9 @@ const migrations = [
   `
     ALTER TABLE settings ADD COLUMN showSets BOOLEAN DEFAULT 1
   `,
+  `
+    CREATE INDEX sets_created ON sets(created)
+  `,
 ];
 
 export let db: SQLiteDatabase;