All previously-nullable account_id columns are now NOT NULL.
tree_embeddings and feedback backfilled before constraint applied.
Global content assigned to platform sentinel account (00000000-...-0001)
in preceding migration.
Tables updated: users, trees, tree_categories, tree_tags,
step_categories, step_library, tree_embeddings, feedback
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
