feat(accounting): snapshot vat_code on quotes/invoices + export prefers it (foundation)

First slice of the VAT-consolidation: migration 130 adds a nullable vat_code
snapshot column to quotes + invoices, and the Treuhänder export now prefers the
invoice's snapshotted code over the (mutable) rate→code map, so a historical
invoice's VatCode never changes when codes are re-mapped. Schema-drift guarded;
behaviour-neutral until the editors start writing the snapshot (next slices).

Part of: VAT registry → Settings→Accounting, invoice VAT dropdown, registration/
reclaim toggle.

backend/migrations/core/130_add_vat_code_snapshot.js

@@ -0,0 +1,36 @@
+/**
+ * Migration: snapshot the chosen VAT code on quotes + invoices.
+ *
+ * The invoice/quote editors now pick an output VAT code from the central
+ * vat_codes registry (Settings → Accounting) instead of free-typing a rate.
+ * We snapshot the CODE STRING (e.g. "UN81") on the document at create time —
+ * alongside the existing vat_rate — so the Treuhänder/accounting export emits
+ * exactly the code the document was issued with, immutably. Editing or deleting
+ * a vat_codes row later never changes a historical document's export code.
+ *
+ *   quotes.vat_code     nullable string (snapshot)
+ *   invoices.vat_code   nullable string (snapshot)
+ *
+ * Legacy rows stay null; the export falls back to the rate→code map for those.
+ * Idempotent: columns guarded.
+ */
+
+exports.up = async function (knex) {
+  for (const tbl of ['quotes', 'invoices']) {
+    if ((await knex.schema.hasTable(tbl)) && !(await knex.schema.hasColumn(tbl, 'vat_code'))) {
+      await knex.schema.alterTable(tbl, (table) => {
+        table.string('vat_code', 16);
+      });
+    }
+  }
+};
+
+exports.down = async function (knex) {
+  for (const tbl of ['quotes', 'invoices']) {
+    if ((await knex.schema.hasTable(tbl)) && (await knex.schema.hasColumn(tbl, 'vat_code'))) {
+      await knex.schema.alterTable(tbl, (table) => {
+        table.dropColumn('vat_code');
+      });
+    }
+  }
+};