snapshot the Pfand rate used for returns, like every other money value already is

Line items correctly snapshot both unit_price_cents and
pfand_cents_per_unit, so a later price change doesn't rewrite history.
Returns didn't get the same treatment: transactions.pfand_returns
stored a bare count, and the euro value came from the bar's
pfand_cents at that moment but was never recorded — change a bar's
deposit mid-event and no historical refund amount could be
recomputed from the database.

Writing returns as transaction_items rows (the schema's is_return
column) doesn't fit cleanly: a Pfand return isn't tied to a specific
drink, but transaction_items.drink_id is NOT NULL. Add
transactions.pfand_cents_at_sale instead — same snapshot idea, at
the transaction level where pfand_returns already lives. Also added
to the transactions CSV export so the recovered value is actually
visible somewhere in the app, not just reachable via raw SQL.

server/src/routes/admin.ts

@@ -267,11 +267,11 @@ export function registerAdminRoutes(app: FastifyInstance, db: DB) {
     let rows: any[];
     let header: string[];
     if (what === 'transactions') {
-      header = ['id', 'bar_id', 'bar_name', 'created_at', 'total_cents', 'crew', 'pfand_returns', 'client_ip', 'client_uuid'];
+      header = ['id', 'bar_id', 'bar_name', 'created_at', 'total_cents', 'crew', 'pfand_returns', 'pfand_cents_at_sale', 'client_ip', 'client_uuid'];
       rows = db
         .prepare(
           `SELECT t.id, t.bar_id, b.name AS bar_name, t.created_at, t.total_cents,
-                  t.crew, t.pfand_returns, t.client_ip, t.client_uuid
+                  t.crew, t.pfand_returns, t.pfand_cents_at_sale, t.client_ip, t.client_uuid
              FROM transactions t JOIN bars b ON b.id = t.bar_id
             ORDER BY t.id`
         )

server/src/routes/public.ts

@@ -135,8 +135,9 @@ export function registerPublicRoutes(app: FastifyInstance, db: DB) {
     const clientIp = req.ip ?? null;
 
     const insertTx = db.prepare(
-      `INSERT INTO transactions (bar_id, created_at, total_cents, crew, pfand_returns, client_ip, client_uuid)
-       VALUES (?, ?, ?, ?, ?, ?, ?)`
+      `INSERT INTO transactions
+         (bar_id, created_at, total_cents, crew, pfand_returns, pfand_cents_at_sale, client_ip, client_uuid)
+       VALUES (?, ?, ?, ?, ?, ?, ?, ?)`
     );
     const insertItem = db.prepare(
       `INSERT INTO transaction_items
@@ -148,7 +149,16 @@ export function registerPublicRoutes(app: FastifyInstance, db: DB) {
     let txId: number;
     try {
       txId = db.transaction(() => {
-        const info = insertTx.run(bar_id, createdAt, paidTotal, crew ? 1 : 0, pfand_returns, clientIp, client_uuid);
+        const info = insertTx.run(
+          bar_id,
+          createdAt,
+          paidTotal,
+          crew ? 1 : 0,
+          pfand_returns,
+          bar.pfand_cents,
+          clientIp,
+          client_uuid
+        );
         const id = Number(info.lastInsertRowid);
         for (const p of priced) {
           insertItem.run(id, p.drink_id, p.qty, p.unit_price_cents, p.pfand_cents_per_unit);