feat(system): allow PublicConfigProvider to replace public config payload

backend/core/contracts/config_public.go

@@ -0,0 +1,12 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package contracts
+
+import "context"
+
+// PublicConfigProvider supplies the payload for GET /api/v1/config/public
+// when a downstream plugin replaces Wavelet's default {configs, app} JSON.
+type PublicConfigProvider interface {
+	PublicConfig(ctx context.Context) (any, error)
+}