Verify and enforce NOT NULL on vfolders.creator

vfolders.creator is recorded on every creation path but is declared nullable. Verify legacy rows, decide the backfill strategy for rows with unknown creators, then migrate to NOT NULL.

For each column: backfill existing NULL rows, ensure an appropriate default/server default, add an Alembic migration setting NOT NULL, and update the ORM model type annotations.

JIRA Issue: BA-7221

Start by locating the vfolders.creator ORM definition, its creation paths, and the related migration history. Inspect legacy rows with NULL creators and determine the backfill strategy before adding the default, NOT NULL migration, and updated type annotations. Done means existing rows are handled and future creations cannot leave creator unset.

Written by the indexing model from the issue text.