fix(migration): rebase down_revision onto 33d7e0e21daa (sc-105349)

CI cypress + playwright shards were red with:

  ERROR [flask_migrate] Error: Multiple head revisions are present
  for given argument 'head'

The recent rebase onto master pulled in
``33d7e0e21daa_add_semantic_layers_and_views.py`` (from PR #37815,
"semantic layer extension"), which had been authored against
``ce6bd21901ab`` as its parent — the same parent our migration
referenced. After the rebase both migrations point at
``ce6bd21901ab``, producing two heads and breaking ``flask db
upgrade head`` for any downstream consumer (CI's Cypress / Playwright
shards spin up a real Superset instance via ``superset db upgrade``,
which is why those shards failed first; the integration shards run
against a precomputed schema and didn't surface this).

Fix: chain our migration after the semantic-layer migration by
pointing ``down_revision`` at ``33d7e0e21daa``. The chain is now
linear:

    ... → ce6bd21901ab → 33d7e0e21daa (semantic layers)
                          → 2bee73611e32 (composite PK, this PR)

Verified with ``superset db heads`` (returns single head
``2bee73611e32``) and the local migration test suite (44 passed,
1 skipped).

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