fix(accounting): tax report degrades gracefully if cost side fails (+ surface the error)

The cost side is supplementary — it must never 500 the core revenue report.
getTaxReport now wraps loadCosts in try/catch: on failure it returns empty
costs + a costsError string and logs the real error. The tax page shows the
revenue report plus a non-fatal amber banner with the cost-side error message,
so the actual cause is visible in the UI instead of an opaque 500.

backend/src/services/taxReportService.js

@@ -49,6 +49,7 @@ const REPORTABLE_STATUSES = ['sent', 'paid', 'overdue', 'pending_delivery', 'can
 
 // D.2 — `ensureInt` consolidated into utils/numericHelpers.
 const { ensureInt } = require('../utils/numericHelpers');
+const logger = require('../utils/logger');
 
 function ensureRate(v) {
   if (v === null || v === undefined || v === '') return 0;
@@ -449,10 +450,20 @@ async function getTaxReport({ from, to, currency, includeCosts = true } = {}) {
     const totalsByVatRate = Array.from(byRate.values()).sort((a, b) => a.vatRate - b.vatRate);
 
     // Cost side (Einnahmen-Ausgaben). Optional so legacy callers that
-    // only want the revenue listing can opt out.
+    // only want the revenue listing can opt out. The cost side is
-    const costs = includeCosts
+    // SUPPLEMENTARY — if it fails (e.g. an accounting table/column missing
-      ? await loadCosts({ from, to, cur })
+    // on an older install) it must NOT take down the core revenue report.
-      : { rows: [], totalNet: 0, totalVat: 0, totalGross: 0 };
+    // Degrade to empty costs + log the real error for diagnosis.
+    let costs = { rows: [], totalNet: 0, totalVat: 0, totalGross: 0 };
+    let costsError = null;
+    if (includeCosts) {
+      try {
+        costs = await loadCosts({ from, to, cur });
+      } catch (err) {
+        costsError = err.message;
+        logger.error?.(`taxReport: cost side failed (revenue still returned): ${err.message}`);
+      }
+    }
 
     // Summary: income vs cost vs result. Result = a simplified
     // Einnahmen-Ausgaben surplus (net basis); vatPayable = output VAT
@@ -478,6 +489,7 @@ async function getTaxReport({ from, to, currency, includeCosts = true } = {}) {
       grandTotal,
       cancelledCount,
       costs,
+      costsError,
       summary,
       currency: cur,
       period: { from, to },

frontend/src/pages/admin/clients/TaxReportPage.tsx

@@ -358,6 +358,19 @@ export const TaxReportPage: React.FC = () => {
         )}
       </div>
 
+      {/* Non-fatal: the revenue report loaded but the cost side errored. */}
+      {report?.costsError && (
+        <Card padding="md">
+          <div className="flex items-start gap-3 text-amber-700 dark:text-amber-400">
+            <AlertCircle className="w-5 h-5 flex-shrink-0 mt-0.5" />
+            <div>
+              <p className="font-medium">{t('taxReport.costsErrorTitle', 'Costs could not be loaded')}</p>
+              <p className="text-sm text-neutral-600 dark:text-neutral-400 mt-1 break-words">{report.costsError}</p>
+            </div>
+          </div>
+        </Card>
+      )}
+
       {/* Results */}
       {isLoading ? (
         <Card padding="lg"><Loading /></Card>

frontend/src/services/taxReport.service.ts

@@ -97,6 +97,8 @@ export interface TaxReport {
   /** Cost side (#4). Present when the accounting tables exist; empty
    *  otherwise. */
   costs: TaxReportCosts;
+  /** Non-fatal: set when the cost side failed to load (revenue still shown). */
+  costsError?: string | null;
   /** Income/cost/result summary (#4). */
   summary: TaxReportSummary;
   currency: string;