uGig OpenAPI invoice wallet fields patch

This fixes a public API contract gap found while creating worker invoices through the documented API. POST /api/gigs/{id}/invoice rejects requests unless a CoinPay receiving currency and wallet are included, but the OpenAPI request schema only required application_id and amount.

Validation passed: request schema now includes payment_currency and merchant_wallet_address; npm run test:run -- src/app/api/openapi.json/spec.test.ts passed with 9 tests.

Patch

diff --git a/public/openapi.json b/public/openapi.json
index 3ffd771..7bf71d3 100644
--- a/public/openapi.json
+++ b/public/openapi.json
@@ -1941,11 +1941,24 @@
             "application/json": {
               "schema": {
                 "type": "object",
-                "required": ["application_id", "amount"],
+                "required": [
+                  "application_id",
+                  "amount",
+                  "payment_currency",
+                  "merchant_wallet_address"
+                ],
                 "properties": {
                   "application_id": { "type": "string", "format": "uuid" },
                   "amount": { "type": "number", "exclusiveMinimum": 0 },
                   "currency": { "type": "string", "default": "USD" },
+                  "payment_currency": {
+                    "type": "string",
+                    "description": "CoinPay receiving currency from /api/coinpay/wallets, such as sol or eth."
+                  },
+                  "merchant_wallet_address": {
+                    "type": "string",
+                    "description": "Worker's CoinPay global wallet address for the selected payment_currency."
+                  },
                   "notes": { "type": "string" },
                   "due_date": { "type": "string", "format": "date" }
                 }