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/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 /