feat(crm): per-customer Skonto opt-out

Adds customer_accounts.skonto_disabled (migration 112) so a customer
that negotiated "no early-payment discount" can be flagged once instead
of ticking the per-invoice toggle on every invoice. resolveSkontoPercent
ForInvoice and the PDF render context both honour it, extending the
resolution chain to customer → invoice → snapshot → quote → global.
Checkbox added to the customer detail Billing card (en + de).

frontend/src/i18n/locales/de.json

@@ -3134,6 +3134,8 @@
       "quarterly": "Quartalsweise",
       "cycleDay": "Stichtag",
       "cycleDayHint": "1..28 = Tag im Monat. Negativ -1..-15 für „N Tage vor Monatsende“ (so löst -3 in einem 31-Tage-Monat am 28. aus).",
+      "skontoDisabled": "Kein Skonto für diesen Kunden",
+      "skontoDisabledHint": "Deaktiviert den Skonto-Abzug auf allen Rechnungen dieses Kunden – unabhängig von Vorlage oder globalen Standardwerten.",
       "triggerNow": "Rechnung jetzt ausstellen",
       "triggerConfirm": "Monatsrechnung für diesen Kunden jetzt ausstellen? Der Kunde erhält die E-Mail sofort.",
       "triggerHint": "Überspringt den Stichtag und stellt den aktuellen Entwurf sofort aus. Wird abgelehnt, wenn für die aktuelle Periode nichts erfasst wurde.",

frontend/src/i18n/locales/en.json

@@ -3134,6 +3134,8 @@
       "quarterly": "Quarterly",
       "cycleDay": "Cycle day",
       "cycleDayHint": "1..28 = day of month. Use negative -1..-15 for \"N days before month end\" (so -3 fires on the 28th of a 31-day month).",
+      "skontoDisabled": "No Skonto for this customer",
+      "skontoDisabledHint": "Disables the early-payment discount on all of this customer’s invoices, regardless of template or global defaults.",
       "triggerNow": "Trigger invoice now",
       "triggerConfirm": "Issue this customer's monthly bill now? The customer receives the email immediately.",
       "triggerHint": "Bypasses the cadence day and issues the running draft immediately. Refuses when nothing has been queued for the current period.",

frontend/src/pages/admin/CustomerDetailPage.tsx

@@ -40,7 +40,7 @@ type EditableFields =
   | 'addressLine1' | 'addressLine2' | 'postalCode' | 'city' | 'state'
   | 'countryCode' | 'countryName' | 'preferredLanguage' | 'notes'
   | 'featureCalendar' | 'featureQuotes' | 'featureBills' | 'featureHoursLogging'
-  | 'hourlyRateMinor' | 'billingCadence' | 'billingCycleDay';
+  | 'hourlyRateMinor' | 'billingCadence' | 'billingCycleDay' | 'skontoDisabled';
 
 // `fmtDate` (from useLocalizedDate, below) is the single canonical date
 // formatter. It honors the admin's `general_date_format` setting AND
@@ -140,6 +140,7 @@ export const CustomerDetailPage: React.FC = () => {
         hourlyRateMinor: customer.hourlyRateMinor ?? null,
         billingCadence: customer.billingCadence ?? 'per_event',
         billingCycleDay: customer.billingCycleDay ?? 1,
+        skontoDisabled: customer.skontoDisabled ?? false,
       } as any);
     }
   }, [customer, form]);
@@ -738,6 +739,25 @@ export const CustomerDetailPage: React.FC = () => {
           )}
         </div>
 
+        {/* Per-customer Skonto opt-out (migration 112). For B2B
+            customers who negotiated "no early-payment discount" — set
+            once instead of ticking the per-invoice toggle every time. */}
+        <label className="mt-4 flex items-start gap-2 text-sm text-theme">
+          <input
+            type="checkbox"
+            checked={!!form.skontoDisabled}
+            onChange={(e) => setForm((prev) => ({ ...prev, skontoDisabled: e.target.checked } as any))}
+            className="mt-0.5 rounded border-neutral-300 dark:border-neutral-600"
+          />
+          <span>
+            {t('customers.billing.skontoDisabled', 'No Skonto for this customer')}
+            <span className="block text-xs text-muted-theme">
+              {t('customers.billing.skontoDisabledHint',
+                'Disables the early-payment discount on all of this customer’s invoices, regardless of template or global defaults.')}
+            </span>
+          </span>
+        </label>
+
         {/* Preview of the open monthly draft (migration 128). Shows
             every line item queued for the customer's current billing
             period so admin sees exactly what "Trigger invoice now"

frontend/src/services/customerAdmin.service.ts

@@ -60,6 +60,10 @@ export interface CustomerAccountDetail extends CustomerAccountSummary {
    */
   billingCadence?: 'per_event' | 'monthly' | 'quarterly';
   billingCycleDay?: number;
+  /** Per-customer Skonto opt-out (migration 112). When true, none of
+   *  this customer's invoices qualify for an early-payment discount,
+   *  regardless of template / global defaults. */
+  skontoDisabled?: boolean;
   notes: string | null;
   events: Array<{
     id: number;
@@ -158,6 +162,8 @@ export const customerAdminService = {
       // CRM billing cadence (migration 102 + 128).
       billingCadence: 'billing_cadence',
       billingCycleDay: 'billing_cycle_day',
+      // Per-customer Skonto opt-out (migration 112).
+      skontoDisabled: 'skonto_disabled',
     };
     for (const [k, v] of Object.entries(payload)) {
       if (k in map) snake[map[k]] = v;