The amount on a received invoice is the GROSS total the user pays (VAT
included), and VAT is the portion contained within it — not a net base with
VAT added on top. Switch the formula from amount*rate/100 to the
VAT-inclusive amount*rate/(100+rate) via a shared, tested vatFromGross()
helper used by both the create (manual upload + AI import share this
endpoint) and edit paths. Rate 0 → no VAT.
- AI extraction now asks Claude for the total INCLUDING VAT (was "základ bez
DPH"), reinforced in the structured-output schema description.
- Label the amount field "Částka s DPH" in the AI review card and the manual
upload/edit forms so the gross convention is explicit.
- Add unit tests with the user-confirmed figures (22 542,91 @ 21% → 3 912,41,
base 18 630,50) plus other rates and the no-VAT case.
Existing rows keep their stored (gross) amounts; their vat_amount recomputes
on next edit. A one-shot backfill can correct historical vat_amount if wanted.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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