add golangci-lint to pipeline and fix lint issues

- Add golangci-lint-action to GitHub Actions workflow
- Fix invoice state check in LND backend (use inv.State == SETTLED)
- Handle unused fmt.Fprintf return value in handler

Author: qustavo

.github/workflows/go.yml

@@ -21,6 +21,9 @@ jobs:
       with:
         go-version: '1.24'
 
+    - name: golangci-lint
+      uses: golangci/golangci-lint-action@v6
+
     - name: Build
       run: go build -v ./...
 

pkg/lightning/lnd.go

@@ -111,5 +111,5 @@ func (b *LNDBackend) VerifyPayment(ctx context.Context, paymentHash string) (boo
 	if err != nil {
 		return false, fmt.Errorf("LookupInvoice: %w", err)
 	}
-	return inv.Settled, nil
+	return (inv.State == lnrpc.Invoice_SETTLED), nil
 }

pkg/proxy/handler.go

@@ -58,7 +58,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			h.log.Warn("invalid L402 credentials", "err", err, "path", r.URL.Path)
 			w.Header().Set("Content-Type", "application/json")
 			w.WriteHeader(http.StatusBadRequest)
-			fmt.Fprintf(w, `{"error":%q}`, err.Error())
+			_, _ = fmt.Fprintf(w, `{"error":%q}`, err.Error())
 			return
 		}
 		if err := h.validateAuth(r, token, preimage); err == nil {