Fix `cobra_match` so that `spot_id` can be `NULL`

The primary key of `cobra_match` table is currently (`mcs_frame_id`, `spot_id`). This causes a trouble when tracking and flagging if targets are behind dots or non-detection for some reasons, where they have no `spot_id`. The proposed fix is to change the primary key to (`pfs_visit_id`, `iteration`, `cobra_id`), which is the same as `cobra_target`.