Fix red develop 84efcb8ff01b

The merge lane found `develop` red at `84efcb8ff01bddab1f754dfbe66e8e36bb572b86` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `tests/test_projects_routes.py::test_put_declaring_gitlab_repairs_a_legacy_unknown_host_row` - `api-unit-dc-test` The whole gate log: artifact `10239` on lane task 10187. Make `develop` pass its full gate again. Open a PR as usual: the merge lane merges a PR that fixes some of these and adds no failure even while `develop` is red.

Fixed red develop: legacy unknown-host test now seeds its backfilled project_repos primary row (adopt_legacy was removed in Task 10169). Test-only; 219 tests + quick gate green; PR #6961 queued in merge lane. Memory: none applied.