refactor(auth): modularize auth plugin with physical subpackages and decoupled services

backend/plugins/domain/auth/model/do/oauth_state.go

@@ -0,0 +1,33 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package do provides domain data objects for the auth plugin.
+package do
+
+import "encoding/json"
+
+// OAuthStatePayload represents the cached state verification payload for OAuth flow.
+type OAuthStatePayload struct {
+	SourceName  string `json:"source_name"`
+	Purpose     string `json:"purpose"`
+	UserID      uint64 `json:"user_id,omitempty"`
+	SessionHash string `json:"session_hash"`
+}
+
+// Encode converts OAuthStatePayload to a JSON string.
+func (p OAuthStatePayload) Encode() (string, error) {
+	data, err := json.Marshal(p)
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}
+
+// DecodeOAuthStatePayload parses a JSON string into OAuthStatePayload.
+func DecodeOAuthStatePayload(value string) (OAuthStatePayload, error) {
+	var payload OAuthStatePayload
+	if err := json.Unmarshal([]byte(value), &payload); err != nil {
+		return OAuthStatePayload{}, err
+	}
+	return payload, nil
+}