security: harden plugin sandbox stdin and opaque internal errors

- sandbox_wazero.go: explicitly discard plugin stdin via WithStdin to
  prevent WASM modules from reading the server process's stdin fd
- channel_handler.go: replace ErrInternal message pass-through with
  generic 'an internal error occurred' — full error stays server-side
  in slog.Error only
- invite_handler_test.go: update assertions to expect generic message

Server/api/channel_handler.go

@@ -286,8 +286,7 @@ func writeServiceError(w http.ResponseWriter, err error) {
 		writeJSON(w, http.StatusConflict, errorResponse{Error: "CONFLICT", Message: err.Error()})
 	case errors.Is(err, service.ErrInternal):
 		slog.Error("service error", "err", err)
-		msg := strings.TrimPrefix(err.Error(), "internal error: ")
-		writeJSON(w, http.StatusInternalServerError, errorResponse{Error: "INTERNAL_ERROR", Message: msg})
+		writeJSON(w, http.StatusInternalServerError, errorResponse{Error: "INTERNAL_ERROR", Message: "an internal error occurred"})
 	default:
 		slog.Error("service error", "err", err)
 		writeJSON(w, http.StatusInternalServerError, errorResponse{Error: "INTERNAL_ERROR", Message: "internal error"})

Server/api/invite_handler_test.go

@@ -155,8 +155,8 @@ func TestCreateInvite_CreateInviteFailure(t *testing.T) {
 	if err := json.NewDecoder(rr.Body).Decode(&resp); err != nil {
 		t.Fatalf("decode response: %v", err)
 	}
-	if resp["message"] != "failed to create invite" {
-		t.Errorf("message = %v, want failed to create invite", resp["message"])
+	if resp["message"] != "an internal error occurred" {
+		t.Errorf("message = %v, want an internal error occurred", resp["message"])
 	}
 }
 
@@ -185,8 +185,8 @@ func TestCreateInvite_GetInviteFailure(t *testing.T) {
 	if err := json.NewDecoder(rr.Body).Decode(&resp); err != nil {
 		t.Fatalf("decode response: %v", err)
 	}
-	if resp["message"] != "failed to retrieve invite" {
-		t.Errorf("message = %v, want failed to retrieve invite", resp["message"])
+	if resp["message"] != "an internal error occurred" {
+		t.Errorf("message = %v, want an internal error occurred", resp["message"])
 	}
 	if _, err := database.Exec(`DROP TRIGGER delete_invite_after_insert`); err != nil {
 		t.Fatalf("drop trigger: %v", err)
@@ -387,8 +387,8 @@ func TestRevokeInvite_RevokeFailure(t *testing.T) {
 	if err := json.NewDecoder(rr2.Body).Decode(&resp); err != nil {
 		t.Fatalf("decode revoke failure response: %v", err)
 	}
-	if resp["message"] != "failed to revoke invite" {
-		t.Errorf("message = %v, want failed to revoke invite", resp["message"])
+	if resp["message"] != "an internal error occurred" {
+		t.Errorf("message = %v, want an internal error occurred", resp["message"])
 	}
 }
 

Server/plugin/sandbox_wazero.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"strings"
 
 	"github.com/tetratelabs/wazero"
 	"github.com/tetratelabs/wazero/api"
@@ -76,6 +77,7 @@ func (r *Registry) activateWithRuntime(ctx context.Context, inst *Instance) erro
 
 	modCfg := wazero.NewModuleConfig().
 		WithName(inst.Manifest.Name).
+		WithStdin(strings.NewReader("")). // prevent plugins from reading server stdin
 		WithStdout(io.Discard).
 		WithStderr(io.Discard).
 		WithStartFunctions() // suppress _start; exports are called on demand