feat(versioning): foundation — Continuum capture + parent/child shadow tables + VersionDAO

Adds SQLAlchemy-Continuum as a dependency and wires it as the
canonical capture mechanism for chart, dashboard, and dataset edits.

**Schema** — three Alembic migrations, leaving the chain at one
foundation revision plus one child-shadow revision:

- ``version_transaction`` (renamed from Continuum's default
  ``transaction``; SQL-reserved-word workaround) carries the per-save
  ``user_id`` / ``issued_at`` and is the join target for all shadow
  rows. Auto-incrementing PK; user_id has no FK so import / Celery /
  CLI saves can write rows without an active Flask user.
- Parent shadow tables for the three entity types:
  ``dashboards_version``, ``slices_version``, ``tables_version``.
- Child shadow tables for dataset children + dashboard M2M:
  ``table_columns_version``, ``sql_metrics_version``,
  ``dashboard_slices_version`` (composite PK on the M2M shadow,
  matching the live ``dashboard_slices`` reshape from
  sc-105349-composite-association-pks).

**Models** — ``Dashboard``, ``Slice``, ``SqlaTable`` (and dataset
children ``TableColumn`` / ``SqlMetric``) gain ``__versioned__``
class attributes. The exclude lists carry both M2M relationships
(``owners``, ``roles``, ``dashboards``) and the ``AuditMixin``
columns (``changed_on`` / ``created_on`` / ``changed_by_fk`` /
``created_by_fk`` plus ``last_saved_at`` / ``last_saved_by_fk``
on ``Slice``) so auto-bumped audit fields cannot trigger a
version row on their own (FR-025).

**Plugins** — ``superset/versioning/factory.py`` ships three
Continuum plugins:

- ``VersionTransactionFactory`` renames the transaction table and
  appends the unconditional ``user_id`` column.
- ``VersioningFlaskPlugin`` sources the acting user from Superset's
  ``g.user`` rather than ``flask_login.current_user`` (Superset's
  JWT auth populates ``g.user`` but leaves ``current_user``
  anonymous on API routes).
- ``SkipUnmodifiedPlugin`` filters Continuum's UPDATE operations,
  marking content-equivalent re-saves as ``processed=True`` so they
  don't mint no-op shadow rows (FR-026; see follow-up commits for
  the test). Lives in this commit because it shares the file with
  the other plugins.

**Save-path glue** — a ``before_flush`` baseline listener
(``superset/versioning/baseline.py``) inserts an ``operation_type=0``
shadow row the first time a pre-existing entity is saved, including
the slice-baseline-under-dashboard pattern that gives the dashboard
M2M shadow a row to join against. ``UpdateDashboardCommand`` wraps
its body in ``no_autoflush`` so ``process_tab_diff`` /
``process_native_filter_diff`` don't fire intermediate flushes that
would mint extra version rows. ``DatasetDAO.update_columns`` is
rewritten as a natural-key upsert keyed on ``column_name`` so child
edits flow through ORM events Continuum sees.

**DAO** — ``superset/daos/version.py`` exposes the read API used by
the version endpoints in the next commits:
``current_version_number`` (0-based index, unstable under retention
pruning), ``current_live_transaction_id`` (stable across pruning),
``current_live_version_uuid`` (deterministic UUIDv5), plus
``list_versions`` / ``get_version`` / ``restore_version`` and a
batch ``list_change_records_batch`` for N+1 avoidance.

**Initialization** — ``superset/initialization/__init__.py`` wires
``init_versioning()`` after ``make_versioned()`` runs and the
versioned mappers are configured. Registers the baseline listener
plus the change-record listener (the latter's body lives in the
next commit but the registration site is here because it shares
the init function).

**Tests** — version-capture and version-list integration tests for
each entity type, plus a ``VersionDAO`` unit test suite. Retention
test uses a backdated ``issued_at`` so it can drive
``_prune_old_versions_impl`` synchronously.

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