flaky api tests: git clone from shared full-data fixture repo loses objects mid-copy under parallel workers

Seen twice on 2026-09-09 in CI on the same commit (`96b71ef`, merge of #157, which touched only `apps/web`):

- push run: `tests/slug-redirect.test.ts > never intercepts /api/* paths`
- pull_request run: `tests/avatar-upload.test.ts > 404s for a missing person` and `> rejects unsupported MIME types with 422`

All with the same error from `apps/api/tests/helpers/seed-fixtures.ts` cloning the bare fixture repo created by `tests/helpers/test-full-repo.ts`:

```
Error: Command failed: git clone /tmp/cfp-full-data-XXXX/data.git /tmp/cfp-seed-wt-YYYY
fatal: failed to copy file to '/tmp/cfp-seed-wt-YYYY/.git/objects/71/fdd3...': No such file or directory
```

Different test files, different temp dirs, same shape: a local `git clone` (hardlink/copy of loose objects) races with something mutating the source repo's object store while it runs. Suspects: another worker's transaction on the same shared fixture repo (gitsheets 2.x Rust core landed on `develop` via #151 and may repack/prune loose objects), or a cleanup `rm` in a sibling file's `afterEach`. Locally the suite passes with `--maxWorkers=2`; CI uses the default worker count.

Options: give each test file its own fixture repo (no shared source), clone with `--no-hardlinks`/`file://` (forces a pack transfer instead of per-object copies), or serialize the clone behind a lock. Re-running the job passes, so this is a flake, not a regression, but it will keep failing release CI at random.

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