preimages: support fiat prices, converted at invoice-creation time

POST /preimages now accepts fiat + optional currency (defaults to the
merchant's) as an alternative to a sat price. The fiat amount is stored
per entry and converted to sats at the moment a usePreimage invoice is
created, so the price tracks the exchange rate instead of freezing at
upload time. The invoice also takes on the entry's currency and rate so
receipts display the fiat price exactly as the merchant set it. If no
rate is available for the entry's currency, the preimage is pushed back
onto the queue and creation fails rather than issuing an any-amount
invoice.

Co-Authored-By: Claude Fable 5 <[email protected]>

1 files changed, 25 insertions, 8 deletions

diff --git a/routes/preimages.ts b/routes/preimages.ts
index f6632b38..49673d17 100644
--- a/routes/preimages.ts
+++ b/routes/preimages.ts
@@ -1,5 +1,5 @@
 import { createHash } from "node:crypto";
-import { db } from "$lib/db";
+import { db, g } from "$lib/db";
 import { parseEntry } from "$lib/invoices";
 import { bail, fail } from "$lib/utils";
 
@@ -7,9 +7,10 @@ import { bail, fail } from "$lib/utils";
 // with usePreimage: true — each such invoice consumes the next queued preimage
 // so its payment hash is sha256(preimage). Once paid, the preimage is exposed
 // on the invoice record, letting a static storefront sell decryption secrets
-// with no backend of its own. An optional per-batch price (sats) is enforced
-// at invoice creation — invoice creation is unauthenticated, so without it a
-// buyer could name their own price for the secret.
+// with no backend of its own. An optional per-batch price — sats, or a fiat
+// amount converted at invoice-creation time — is enforced at invoice creation;
+// creation is unauthenticated, so without it a buyer could name their own
+// price for the secret.
 
 const MAX_QUEUE = 1000;
 const key = (uid) => `${uid}:preimages`;
@@ -19,7 +20,8 @@ const sha256 = (hex) =>
 export default {
   async add(req, res) {
     try {
-      const { preimages, price = null } = req.body;
+      const { preimages, price = null, fiat = null } = req.body;
+      let { currency = null } = req.body;
       if (!Array.isArray(preimages) || !preimages.length)
         fail("preimages array required");
       if (
@@ -28,8 +30,19 @@ export default {
         )
       )
         fail("preimages must be 64-character hex strings");
+      if (price !== null && fiat !== null)
+        fail("specify price (sats) or fiat, not both");
       if (price !== null && (!Number.isInteger(price) || price <= 0))
         fail("price must be a positive integer amount of sats");
+      if (fiat !== null) {
+        if (typeof fiat !== "number" || !(fiat > 0))
+          fail("fiat must be a positive number");
+        currency ||= req.user.currency;
+        const rates = await g("rates");
+        if (!rates?.[currency]) fail(`unsupported currency ${currency}`);
+      } else {
+        currency = null;
+      }
 
       const k = key(req.user.id);
       const existing = await db.lRange(k, 0, -1);
@@ -47,7 +60,9 @@ export default {
       if (fresh.length)
         await db.rPush(
           k,
-          fresh.map((preimage) => JSON.stringify({ preimage, price })),
+          fresh.map((preimage) =>
+            JSON.stringify({ preimage, price, fiat, currency }),
+          ),
         );
 
       res.send({
@@ -56,6 +71,8 @@ export default {
           preimage,
           hash: sha256(preimage),
           price,
+          fiat,
+          currency,
         })),
       });
     } catch (e) {
@@ -69,8 +86,8 @@ export default {
       res.send({
         queued: entries.length,
         preimages: entries.map((e) => {
-          const { preimage, price } = parseEntry(e);
-          return { preimage, hash: sha256(preimage), price };
+          const { preimage, price, fiat, currency } = parseEntry(e);
+          return { preimage, hash: sha256(preimage), price, fiat, currency };
         }),
       });
     } catch (e) {