remove unique constraint on visit_key alone

migrations/20240317182405_hits.up.sql

@@ -1,7 +1,7 @@
 create table if not exists hits (
   id integer primary key,
   page text not null, -- the slug from the page
-  visit_key text not null unique,
+  visit_key text not null,
   viewed timestamp not null default CURRENT_TIMESTAMP,
   unique(page, visit_key)
 );