Clean up schema for table lfs_objects_projects
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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
- [x] Wrong data type: created_at should be timestamptz
- [x] Wrong data type: updated_at should be timestamptz
- [ ] Missing constraint: created_at NOT NULL
- [ ] Missing constraint: updated_at NOT NULL
- [ ] Missing FK: project_id -> projects
- [ ] Missing FK: lfs_object_id -> lfs_objects
- [ ] Missing constraint: UNIQUE (lfs_object_id, project_id) (1848 entries violate this rule)
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