_ensure_legacy_workspace creates the nil-UUID anchor (slug=legacy)
owned by the platform admin (or oldest user as fallback). Raises a
clear error if the DB has no users at all so we never silently create
an orphaned workspace. Step 3 UPDATEs each table's remaining
workspace_id IS NULL rows to LEGACY_WORKSPACE_ID. Orchestrator wires
ensure-then-loop between Step 2 and Step 3. 3 new tests: orphan
fan-out, no-users error, end-to-end orchestrator with mixed owned +
orphan rows.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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