feat(versioning): time-based retention via Celery beat (FR-007)

Adds a scheduled Celery task that prunes version history older than
``SUPERSET_VERSION_HISTORY_RETENTION_DAYS`` (default 30; settable
via env var; ``0`` disables retention entirely).

**Task** — ``superset.tasks.version_history_retention.prune_old_versions``:

1. Computes ``cutoff = utcnow() - timedelta(days=N)``.
2. Selects ``version_transaction.id`` rows with ``issued_at <
   cutoff`` and filters out any tx whose parent shadow includes a
   live row (``end_transaction_id IS NULL``). The live row is the
   only preservation rule — closed historical rows including the
   baseline (``operation_type=0``) age out. Per-entity minimum-history
   floor is an open question tracked in ``future-work.md``.
3. Deletes rows owned by surviving txs in each parent shadow
   table (``dashboards_version`` / ``slices_version`` /
   ``tables_version``).
4. Deletes child-shadow rows for the same transactions
   (``table_columns_version`` / ``sql_metrics_version`` /
   ``dashboard_slices_version``).
5. Drops the surviving ``version_transaction`` rows. The
   ``version_changes`` rows cascade via the FK from the previous
   commit.

Idempotent and safely retried on partial failure.

**Schedule** — ``superset/config.py`` adds the task to the default
``CeleryConfig.beat_schedule`` (nightly at 03:00). Operators who
override ``CeleryConfig`` in their ``superset_config.py`` need to
merge this entry — see UPDATING.md.

Also adds ``"expose_headers": ["ETag"]`` to the default
``CORS_OPTIONS`` so cross-origin browser clients can read the
``ETag`` header introduced in the next commit. (Co-located here
because both touch ``superset/config.py``; the ETag mechanism
itself ships in the next commit.)

**Auto-discovery** — ``superset/tasks/celery_app.py`` adds
``version_history_retention`` to its late-imports so Celery's
auto-discovery picks up the task.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>