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server/lib/billing.js

@@ -623,6 +623,37 @@ const consumeCreditsWithIdempotency = async (db, userId, key, cost) => {
   });
 };
 
+const refundCredits = (account, amount) => {
+  if (amount <= 0) return 0;
+
+  // Spiegelbildlich zu consumeCredits: zuerst das Monatskontingent entlasten,
+  // ein etwaiger Rest wandert auf das Topup-Guthaben zurück.
+  let remaining = amount;
+  const monthlyRefund = Math.min(account.usedThisCycle, remaining);
+  account.usedThisCycle -= monthlyRefund;
+  remaining -= monthlyRefund;
+
+  if (remaining > 0) {
+    account.topupBalance += remaining;
+  }
+
+  return amount;
+};
+
+const refundCreditsWithIdempotency = async (db, userId, key, amount) => {
+  return runInTransaction(db, async (tx) => {
+    const existing = await readIdempotentValue(tx, key);
+    if (existing && typeof existing.refunded === 'number') return existing.refunded;
+
+    const account = await getOrCreateAccount(tx, userId);
+    const refunded = refundCredits(account, amount);
+    account.updatedAt = nowIso();
+    await upsertAccount(tx, account);
+    await writeIdempotentValue(tx, key, { refunded });
+    return refunded;
+  });
+};
+
 const getBillingSummary = async (db, userId) => {
   if (userId === 'guest') {
     return {
@@ -820,6 +851,7 @@ module.exports = {
   chargeKey,
   claimNotificationOnce,
   consumeCreditsWithIdempotency,
+  refundCreditsWithIdempotency,
   endpointKey,
   ensureBillingSchema,
   getAccountSnapshot,