feat: Implement free issue tracking and scheme discount handling in invoices and Shop POs

Distribution/docs/refactor_docs/order_invoice_lifecycle.md

@@ -95,6 +95,7 @@ InvoiceItem                          — the live billing lines
 ├── FK_InvoiceId, FK_ShopPOItemId, FK_ItemId
 ├── Quantity (int)                   — what is billed now (0..ShopPOItem.Quantity)
 ├── UnitPrice (decimal)
+├── IsFree, FK_SchemeId, SchemeName  — free issue copied from the PO line (UnitPrice 0); see 17_promotions_schemes.md
 └── IsActive, CreatedAt              — from Template
 
 InvoiceVersion                       — immutable snapshot of the invoice
@@ -145,7 +146,8 @@ invoice for the preview, the draft editor and the print tab:
   `*** PRINTED ***`, `qty x price` lines, heavy-ruled TOTAL, Paid/Balance, payments, "Thank you!").
 - Money `Rs. 12,450.00`, quantities up to 3 decimals, dates `dd/MM/yyyy`.
 - Deleted lines are shown **struck through in every status** and excluded from all totals.
-- Not included (no data in the app): company/"From" block, logo, bank details, discount, tax, due date.
+- Scheme discount: shown as Subtotal / Scheme discount / Total (`Invoice.DiscountAmount`), re-derived from the current lines on every edit, print and finalize (see 17_promotions_schemes.md). Free-issue lines get a **Free Issue** column and come after the paid lines.
+- Not included (no data in the app): company/"From" block, logo, bank details, tax, due date.
 
 **Print tab** — `/invoices/{id}/print` (own route group `(print)`, no dashboard chrome). Has a
 paper-size selector (A4 / 80mm / 58mm), sets `@page` with `margin: 0` (so the browser's own
@@ -290,8 +292,9 @@ Draft → Printed → Finalized
    also unresolved. Both cases need explicit cascade rules before implementation.
 6. **Exact adjustment payload for `POST /finalize`** — *partly resolved:* quantity adjustments
    (`{ shopPOItemId, adjustedQuantity }`) are implemented and persisted on the invoice lines.
-   Returns, damage and discounts are still open, as is the payload shape for them. "Free issues" handling (items
-   given to the shop at no charge) is a known case that needs its own field(s) in the payload.
+   Returns and damage are still open, as is the payload shape for them. Scheme discounts and free issues are now handled
+   by the invoice itself (re-evaluated on every edit, print and finalize; free lines only ever shrink automatically) — see
+   [17_promotions_schemes.md](../17_promotions_schemes.md).
 
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