refactor(versioning): split VersionDAO into queries + restore modules

VersionDAO carried five distinct concerns under one class — UUID
derivation, version metadata queries, change-record loading,
single-version snapshot retrieval, and restore orchestration. Bob's
"and" test (the clean-code review flagged this as the next structural
fix after the dead-code purge) gives ~600 lines of "queries about
versioned state of one entity AND the workflow that mutates it."

Splits the read and write sides into purpose-built modules:

- ``superset/versioning/queries.py`` — UUID derivation
  (``VERSION_UUID_NAMESPACE``, ``derive_version_uuid``) + read-side
  helpers (``find_active_by_uuid``, ``current_version_number``,
  ``current_live_transaction_id``, ``current_live_version_uuid``,
  ``list_versions``, ``resolve_version_uuid``, ``get_version``,
  ``list_change_records_batch``). ~475 lines.

- ``superset/versioning/restore.py`` — write-side (``restore_version``,
  ``_stamp_audit_fields_for_restore``, ``_RESTORE_RELATIONS``).
  ~140 lines. Depends only on ``queries.find_active_by_uuid`` and
  ``utils.single_flush_scope``.

- ``superset/daos/version.py`` — collapsed to an ~85-line backward-compat
  façade that re-exports both modules under a single ``VersionDAO``
  class via ``staticmethod`` aliases. The module also re-exports
  ``VERSION_UUID_NAMESPACE`` and ``derive_version_uuid`` at module level
  so the ~10 existing callers (api.py handlers, command classes, the
  ETag emitter, integration tests) don't have to change their imports.
  New code is encouraged to import from the sub-modules directly.

The functions themselves are unchanged byte-for-byte aside from
internal call sites being rewritten from ``VersionDAO.foo`` to the bare
function name (since they now live as module-level functions, not
class methods).

One unit-test mock target moved: ``test_restore_version_returns_none_for_unknown_entity``
now patches ``superset.versioning.restore.find_active_by_uuid`` (the
actual call site) instead of ``VersionDAO.find_active_by_uuid`` (which
is now just an alias).

Each of the three modules now has one reason to change. When the
sc-103157 soft-delete pass adds the ``deleted_at IS NULL`` filter to
``find_active_by_uuid``, it touches only ``queries.py``. When a
per-entity-type restore Strategy replaces the string-keyed
``_RESTORE_RELATIONS`` dispatch, it touches only ``restore.py``.