Partial deliveries against a PO line are the reason qty_outstanding is
dropped rather than wired: a stored counter must be decremented correctly
on every receipt, OCR re-run and correction, and PocketBase has no real
transactions to guarantee that. The sum over delivery_note_items cannot
drift.
Records the relation chain, the two invariants (unmatched lines count
toward nothing; over-delivery is legal), and the fulfilment test cases.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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