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).

backend/migrations/core/112_add_customer_skonto_disabled.js

@@ -0,0 +1,34 @@
+/**
+ * Migration: per-customer Skonto opt-out.
+ *
+ * Background: invoices already carry a per-invoice `skonto_disabled`
+ * flag (migration 126). For B2B customers who negotiated "no early-
+ * payment discount" as a standing contract term, the admin had to tick
+ * that toggle on every single invoice. This adds a customer-level flag
+ * so the opt-out is set once and applies to all of that customer's
+ * invoices. The resolver chain becomes:
+ *   customer.skonto_disabled → invoice.skonto_disabled →
+ *   invoice snapshot → source-quote snapshot → global default.
+ *
+ * Default false so existing customers keep inheriting whatever Skonto
+ * the template / global default offers — no behaviour change on upgrade
+ * (see migration-preserve-existing-state guidance).
+ *
+ * Idempotent: guarded by hasColumn so a re-run is a no-op.
+ */
+
+exports.up = async function(knex) {
+  if (!(await knex.schema.hasTable('customer_accounts'))) return;
+  if (await knex.schema.hasColumn('customer_accounts', 'skonto_disabled')) return;
+  await knex.schema.alterTable('customer_accounts', (table) => {
+    table.boolean('skonto_disabled').notNullable().defaultTo(false);
+  });
+};
+
+exports.down = async function(knex) {
+  if (!(await knex.schema.hasTable('customer_accounts'))) return;
+  if (!(await knex.schema.hasColumn('customer_accounts', 'skonto_disabled'))) return;
+  await knex.schema.alterTable('customer_accounts', (table) => {
+    table.dropColumn('skonto_disabled');
+  });
+};