feat(crm): allow negative line items for manual discount/Rabatt rows

Drops the isInt({ min: 0 }) constraint on lineItems.*.unitPriceMinor
in both the adminInvoices and adminQuotes POST/PUT validators so
admins can add Treuerabatt / Frühbucherrabatt rows as standalone
negative-priced lines (matches standard DE/CH invoice practice).

A service-layer guard rejects saves whose computed total goes below
zero (INVOICE_TOTAL_NEGATIVE / QUOTE_TOTAL_NEGATIVE, both 400) so a
mis-typed discount can't accidentally mint a credit-balance invoice
that would masquerade as a regular row in dashboards. Credit notes
still belong in the Storno path (createStorno), which is unchanged.

Quote-side integration coverage is omitted for now — createQuote's
cold-require path takes ~30s under the test harness; the invoice
test exercises the same validator + guard shape.

backend/src/routes/adminInvoices.js

@@ -247,7 +247,11 @@ const INVOICE_BODY_VALIDATORS = [
   body('lineItems').optional({ values: 'falsy' }).isArray(),
   body('lineItems.*.description').optional({ values: 'falsy' }).isString().isLength({ min: 1, max: 1000 }),
   body('lineItems.*.quantity').optional({ values: 'falsy' }).isFloat({ min: 0 }),
-  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt({ min: 0 }),
+  // Negative unit prices are allowed so admins can add manual
+  // discount / Rabatt lines (e.g. "Treuerabatt -50,00 €"). The
+  // service-layer total guard rejects invoices whose net goes below
+  // zero — for credit notes, use Storno instead.
+  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt(),
   body('lineItems.*.discountPercent').optional({ values: 'falsy' }).isFloat({ min: 0, max: 100 }),
   // Migration 119: sub-item + details support. Cross-row constraints
   // (parent must exist, max 1 level deep) are enforced by the service
@@ -663,6 +667,16 @@ router.put(
       updates.shipping_amount_minor = shipping;
       updates.total_amount_minor = net + vatAmount + shipping;
 
+      // Negative line items (Rabatt) are allowed, but the resulting
+      // invoice total must not go below zero. Credit notes belong in
+      // the Storno path, not in regular invoice edits.
+      if (updates.total_amount_minor < 0) {
+        return res.status(400).json({
+          error: 'Invoice total cannot be negative. To issue a credit note, cancel the original invoice with Storno.',
+          code: 'INVOICE_TOTAL_NEGATIVE',
+        });
+      }
+
       const quoteService = require('../services/quoteService');
       const { validateLineItemHierarchy, insertLineItemsHierarchical } = quoteService._internal;
       await db.transaction(async (trx) => {

backend/src/routes/adminQuotes.js

@@ -335,7 +335,11 @@ const QUOTE_BODY_VALIDATORS = [
   body('lineItems').optional({ values: 'falsy' }).isArray(),
   body('lineItems.*.description').optional({ values: 'falsy' }).isString().isLength({ min: 1, max: 1000 }),
   body('lineItems.*.quantity').optional({ values: 'falsy' }).isFloat({ min: 0 }),
-  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt({ min: 0 }),
+  // Negative unit prices are allowed so admins can add manual
+  // discount / Rabatt lines (e.g. "Treuerabatt -50,00 €"). The
+  // service-layer total guard rejects quotes whose net goes below
+  // zero — see quoteService.
+  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt(),
   body('lineItems.*.discountPercent').optional({ values: 'falsy' }).isFloat({ min: 0, max: 100 }),
   // Migration 119: sub-item + details support. Cross-row constraints
   // (parent must exist, max 1 level deep) are enforced by the service

backend/src/services/invoiceService.js

@@ -696,6 +696,18 @@ async function createInvoice(payload, adminId, trx = db) {
   const shippingMinor = ensureInt(payload.shippingAmountMinor);
   const totalMinor = netMinor + vatMinor + shippingMinor;
 
+  // Negative line items (Rabatt) are allowed, but the resulting
+  // invoice total must not go below zero. Credit notes belong in
+  // the Storno path (createStorno), which mints a separate
+  // kind='storno' record with cancels_invoice_id set.
+  if (totalMinor < 0) {
+    throw new AppError(
+      'Invoice total cannot be negative. To issue a credit note, cancel the original invoice with Storno.',
+      400,
+      'INVOICE_TOTAL_NEGATIVE',
+    );
+  }
+
   const bank = await businessProfileService.resolveBankAccountForCurrency(currency, payload.businessBankAccountId);
 
   // Snapshot the selected payment-term template (net days / Skonto /

backend/src/services/quoteService.js

@@ -476,6 +476,17 @@ async function createQuote(payload, adminId) {
     payload.shippingAmountMinor
   );
 
+  // Negative line items (Rabatt) are allowed, but the resulting
+  // quote total must not go below zero — a quote represents an
+  // offer of value, not a credit note.
+  if (totals.totalAmountMinor < 0) {
+    throw new AppError(
+      'Quote total cannot be negative. Reduce the discount amount.',
+      400,
+      'QUOTE_TOTAL_NEGATIVE',
+    );
+  }
+
   // Resolve bank account for the chosen currency.
   const bank = await businessProfileService.resolveBankAccountForCurrency(currency, payload.businessBankAccountId);
 
@@ -586,6 +597,16 @@ async function updateQuote(id, payload, adminId) {
     payload.shippingAmountMinor ?? existing.shipping_amount_minor
   );
 
+  // Negative line items (Rabatt) are allowed, but the resulting
+  // quote total must not go below zero. See createQuote.
+  if (totals.totalAmountMinor < 0) {
+    throw new AppError(
+      'Quote total cannot be negative. Reduce the discount amount.',
+      400,
+      'QUOTE_TOTAL_NEGATIVE',
+    );
+  }
+
   return await db.transaction(async (trx) => {
     const updates = {
       updated_at: new Date(),