Update backups table cron_expression column

internal/database/migrations/20240720060503_add_backups_table.sql

@@ -6,7 +6,7 @@ CREATE TABLE IF NOT EXISTS backups (
   destination_id UUID NOT NULL REFERENCES destinations(id) ON DELETE CASCADE,
 
   name TEXT NOT NULL,
-  cron_expression TEXT NOT NULL UNIQUE,
+  cron_expression TEXT NOT NULL,
   time_zone TEXT NOT NULL,
   is_active BOOLEAN NOT NULL DEFAULT FALSE,
   dest_dir TEXT NOT NULL,