See https://gitlab.com/gitlab-org/gitlab/-/issues/476575#note_2301971082 for more details - [x] Add `namespace_id` column to the `redirect_routes` table. - [x] Add index and FK constraint to the column. - [x] Add a trigger/save callback to populate the sharding key on save. - [x] Queue backfill for the sharding key. - [x] Finalize the backfill for the sharding key on %"18.2" - [ ] Add NOT NULL constraints and specify the sharding key on `redirect_routes.yml` Read [GitLab Cells Development Guidelines](https://docs.gitlab.com/ee/development/cells/#choose-either-the-gitlab_main_cell-or-gitlab_main_clusterwide-schema) for more details on how to work with sharding keys.