Creating a dataset, use case or collaborative fails if another is created in the same second #199

What: creating a dataset, use case or collaborative fails when another one is created in the same second, by any user. The default title is New dataset 17 Sep 2026 - 09:43:04 (one-second precision), the slug is derived from it, and slug is globally unique, so the second save hits the constraint and the mutation returns success: False with no error.

Why it matters: a real user can hit it, and it blocked both release gates today when they ran concurrently.

Evidence: release gates for DataSpaceBackend #198 and DataSpaceFrontend #468, api-smoke started 09:43:06 and 09:43:04 — addDataset returned success=False: {'success': False, 'data': None}.

Fix: not fixed — make the generated slug collision-safe (api/schema/dataset_schema.py:587, usecase_schema.py:296/304, collaborative_schema.py:303/311, and the models' save()).