invoices: sell merchant-supplied preimages via a usePreimage queue

Merchants POST /preimages (auth) to pre-load 32-byte hex preimages into a
FIFO queue; creating an invoice with usePreimage: true consumes the next
one so the invoice's payment hash is sha256(preimage). Paying the invoice
reveals the merchant-chosen secret to the buyer — via their own wallet on
external settlements, and via GET /invoice/:id once settled — enabling
backend-less digital download stores (encrypted files on static hosting,
preimage + public string = decryption key).

- GET /preimages lists the queue with payment hashes so the merchant can
  map files to future invoices; POST /preimages/delete clears it
- empty queue fails invoice creation rather than issuing a random
  preimage the buyer would pay for but couldn't decrypt with
- preimages cln rejects as already used are discarded and the next tried
- consumed preimages persist at preimage:${payment_hash} (TTL = invoice
  expiry) and credit() prefers them as the settlement ref, so internal
  coinos-to-coinos payments reveal the real preimage instead of the
  sender's uid

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

1 files changed, 5 insertions, 0 deletions

diff --git a/index.ts b/index.ts
index 0a3bfd89..3a123e9e 100644
--- a/index.ts
+++ b/index.ts
@@ -18,6 +18,7 @@ import lnurl from "$routes/lnurl";
 import locations from "$routes/locations";
 import nostr from "$routes/nostr";
 import payments from "$routes/payments";
+import preimages from "$routes/preimages";
 import rates from "$routes/rates";
 import shopify from "$routes/shopify";
 import square from "$routes/square";
@@ -52,6 +53,10 @@ app.post("/invoice", optional, invoices.create);
 app.post("/invoice/:id", optional, invoices.update);
 app.post("/sign", auth, invoices.sign);
 
+app.get("/preimages", auth, preimages.list);
+app.post("/preimages", auth, preimages.add);
+app.post("/preimages/delete", auth, preimages.clear);
+
 app.get("/assetlinks.json", (_, res) => res.sendFile("assetlinks.json"));
 app.get("/nostr.json", nostr.identities);
 app.get("/profile/:profile", nostr.profile);