fix(payment): fully URL-decode EasyPay callback params before signature verification

Upstream proxy double-encodes GET query params. url.ParseQuery only
decodes once, leaving values like %E5%BF%AB instead of actual UTF-8.
Add fullyDecodeURL to repeatedly decode until stable before signing.

backend/cmd/server/VERSION

@@ -1 +1 @@
-0.1.108.80
+0.1.108.81

backend/internal/handler/payment_webhook_handler.go

@@ -57,9 +57,8 @@ func (h *PaymentWebhookHandler) StripeWebhook(c *gin.Context) {
 func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string) {
 	var rawBody string
 	if c.Request.Method == http.MethodGet {
-		// GET callbacks (e.g. EasyPay): RawQuery may be double-encoded by
-		// upstream proxies, so rebuild from the already-decoded Query().
-		rawBody = c.Request.URL.Query().Encode()
+		// GET callbacks (e.g. EasyPay) pass params as URL query string
+		rawBody = c.Request.URL.RawQuery
 	} else {
 		body, err := io.ReadAll(io.LimitReader(c.Request.Body, maxWebhookBodySize))
 		if err != nil {

backend/internal/payment/provider/easypay.go

@@ -171,7 +171,7 @@ func (e *EasyPay) VerifyNotification(_ context.Context, rawBody string, _ map[st
 	}
 	params := make(map[string]string)
 	for k := range values {
-		params[k] = values.Get(k)
+		params[k] = fullyDecodeURL(values.Get(k))
 	}
 	sign := params["sign"]
 	if sign == "" {
@@ -268,3 +268,15 @@ func easyPaySign(params map[string]string, pkey string) string {
 func easyPayVerifySign(params map[string]string, pkey string, sign string) bool {
 	return hmac.Equal([]byte(easyPaySign(params, pkey)), []byte(sign))
 }
+
+// fullyDecodeURL repeatedly URL-decodes a string until stable.
+// Handles double (or multi) encoding caused by upstream proxies.
+func fullyDecodeURL(s string) string {
+	for {
+		decoded, err := url.QueryUnescape(s)
+		if err != nil || decoded == s {
+			return s
+		}
+		s = decoded
+	}
+}