fix: correct Phase 1 down_revision — chain from 064 not b8d2f4a6c091

b8d2f4a6c091 was NOT the production head. The true head was 064
(064_normalize_script_builder_messages) via the chain:
b8d2f4a6c091 → f0aad74ea51b → 062 → 063 → 064

This caused 'multiple head revisions' on Railway deployment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/versions/cc214c63aa30_add_account_id_core_sessions.py

@@ -9,7 +9,7 @@ from alembic import op
 import sqlalchemy as sa
 
 revision: str = 'cc214c63aa30'
-down_revision: Union[str, None] = 'b8d2f4a6c091'
+down_revision: Union[str, None] = '064'
 branch_labels: Union[str, Sequence[str], None] = None
 depends_on: Union[str, Sequence[str], None] = None