Migration 019 only backfills trees with team_id IS NOT NULL.
Migration 3a40fe11b427 only covered is_default=TRUE trees.
Trees with team_id=NULL and is_default=FALSE (e.g. inactive test trees,
pre-team-system content) fell through both passes and triggered the NULL
guard.
Add two new UPDATE steps after the is_default pass:
1. Assign remaining trees to their author's account (if author has one)
2. Final fallback to PLATFORM_ACCOUNT_ID for any still-NULL rows
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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