Rewrites migration 002 to use ALTER TABLE ... IF EXISTS with per-column guards
and a data-migration DO block that backfills document_json/written_at/batch_id
from the older (tenant_id, data, created_at) layout when present. Updates
GraphChangeStreamProcessor + SavedViewsMigrationHostedService for the aligned
schema and extends the incremental processor tests for the new path.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
d613f8f2a4