add persistent proxy/visitor store with CRUD API and web UI (#5188)

pkg/config/source/source.go

@@ -0,0 +1,37 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package source
+
+import (
+	v1 "github.com/fatedier/frp/pkg/config/v1"
+)
+
+// Source is the interface for configuration sources.
+// A Source provides proxy and visitor configurations from various backends.
+// Aggregator currently uses the built-in config source as base and an optional
+// store source as higher-priority overlay.
+type Source interface {
+	// Load loads the proxy and visitor configurations from this source.
+	// Returns the loaded configurations and any error encountered.
+	// A disabled entry in one source is source-local filtering, not a cross-source
+	// tombstone for entries from lower-priority sources.
+	//
+	// Error handling contract with Aggregator:
+	//   - When err is nil, returned slices are consumed.
+	//   - When err is non-nil, Aggregator aborts the merge and returns the error.
+	//   - To publish best-effort or partial results, return those results with
+	//     err set to nil.
+	Load() (proxies []v1.ProxyConfigurer, visitors []v1.VisitorConfigurer, err error)
+}