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+ * a vat_codes row later never changes a historical document's export code.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');
+      });
+    }
+  }
+};

backend/src/services/ledgerService.js

@@ -254,6 +254,8 @@ async function buildPostings({ from, to, currency } = {}) {
     const postings = [];
 
     // 1) Revenue invoices → Dr Debitoren / Cr Ertrag (gross, output VAT code).
+    // Migration 130 — prefer the snapshotted vat_code; guard for pre-130 DBs.
+    const hasInvVatCode = await db.schema.hasColumn('invoices', 'vat_code');
     const invoices = await db('invoices')
       .leftJoin('customer_accounts', 'invoices.customer_account_id', 'customer_accounts.id')
       .leftJoin('events', 'invoices.event_id', 'events.id')
@@ -263,6 +265,7 @@ async function buildPostings({ from, to, currency } = {}) {
       .orderBy('invoices.issue_date', 'asc')
       .select(
         'invoices.id', 'invoices.invoice_number', 'invoices.issue_date', 'invoices.vat_rate',
+        ...(hasInvVatCode ? ['invoices.vat_code'] : []),
         'invoices.net_amount_minor', 'invoices.vat_amount_minor', 'invoices.total_amount_minor',
         'customer_accounts.company_name as customer_company_name',
         'customer_accounts.first_name as customer_first_name',
@@ -273,7 +276,9 @@ async function buildPostings({ from, to, currency } = {}) {
       );
     for (const inv of invoices) {
       const revAcct = cfg.settings.defaultRevenue;
-      const vatCode = cfg.outputVatMap[rateKey(inv.vat_rate)] || '';
+      // Prefer the snapshot taken at issue time; legacy rows fall back to the
+      // (mutable) rate→code map.
+      const vatCode = inv.vat_code || cfg.outputVatMap[rateKey(inv.vat_rate)] || '';
       const label = buildCustomerLabel(inv);
       postings.push({
         date: inv.issue_date,