feat(gateway): the Robokassa /pay/ edge routes

Add the public /pay/robokassa/result callback proxy (rate-limited; forwards the
provider's form parameters to the backend intake, the single writer, and echoes
its "OK<InvId>" back to Robokassa) and the /pay/robokassa/{success,fail}
browser-return redirects into the app. Route /pay/* to the gateway in the
contour Caddyfile so the callback reaches the edge, not the landing catch-all.
The backend intake now returns the echo body as JSON for the gateway to relay.

gateway/internal/backendclient/api.go

@@ -414,6 +414,20 @@ func (c *Client) WalletOrder(ctx context.Context, userID, productID string) (Wal
 	return out, err
 }
 
+// robokassaResultResp is the backend intake's reply: the body to echo back to Robokassa.
+type robokassaResultResp struct {
+	Response string `json:"response"`
+}
+
+// RobokassaResult forwards a Robokassa Result callback's parameters to the backend intake (the
+// single writer, which verifies the signature and credits) and returns the body to echo to
+// Robokassa ("OK<InvId>"). params carries the provider's raw form fields.
+func (c *Client) RobokassaResult(ctx context.Context, params map[string]string) (string, error) {
+	var out robokassaResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/internal/payments/robokassa/result", "", "", params, &out)
+	return out.Response, err
+}
+
 // CatalogAtomResp is one atom line of a storefront product: the value type it grants and quantity.
 type CatalogAtomResp struct {
 	AtomType string `json:"atom_type"`