For each user with NULL default_workspace_id, generate a base slug
from the email, walk past collisions/blacklist via next_available_slug,
create the workspace + owner membership, and set default_workspace_id.
Idempotent: candidates list is filtered by IS NULL, so re-running on a
populated DB is a no-op. 3 new unit tests (creation, idempotence,
blacklist-walker behaviour).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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