[Domain] Move kernels.domain_name FK to kernels.domain_id

Goal

Move the foreign key reference from ``kernels.domain_name`` (string, → ``domains.name``) to a new ``kernels.domain_id`` (UUID, → ``domains.id``). The legacy ``domain_name`` string column is retained without a FK constraint; removal is deferred to BA-6122. ``domains`` keeps ``name`` as PK in this story — the PK swap is BA-6046.

Scope

1. Alembic: single revision, parent ``down_revision = fdc9d6ac49b4``. Add ``kernels.domain_id`` UUID column (NOT NULL). Backfill via join on ``domains.name``. Drop legacy FK, create new ``domain_id → domains.id`` FK. Downgrade reverses.

2. ORM: add ``domain_id: Mapped[DomainID]`` on ``KernelRow``; keep ``domain_name`` column (drop only its FK clause).

3. Creators: ``KernelRow`` is built from ``KernelSpec`` / parent ``SessionSpec`` in ``repositories/scheduler/creators.py``. Read ``self.spec.scope.domain_id`` from the spec — this depends on the ``SessionScope.domain_id`` field added by BA-6125 (sessions story). If the sessions story has not landed first, this story includes the same ``SessionScope`` / ``SessionScopeDraft`` change as a duplicate; whoever merges second drops the duplicate.

4. Tests: update fixtures that construct ``KernelRow`` directly so ``domain_id`` is populated.

Parallelism

Independent of the other 9 referencing-table stories at the database / FK layer. At the spec layer it shares ``SessionScope.domain_id`` with BA-6125 — merging the two does not conflict because the field is additive.

Out of scope

- ``domains`` PK swap — BA-6046.

- Legacy column drop — BA-6122.

JIRA Issue: BA-6126

Start with the Alembic revision whose parent is fdc9d6ac49b4, then inspect KernelRow and repositories/scheduler/creators.py. Check the SessionScope.domain_id dependency from BA-6125 and update fixtures that construct KernelRow directly. Done means the migration backfills and constrains domain_id, the ORM and creators populate it, and downgrade reverses the database changes.

Written by the indexing model from the issue text.