fix(oauth): 支持 Casdoor 认证，修复第三方回调的用户id为string时会导致服务器内部错误的问题 (#46)

* fix(oauth): support Casdoor string user IDs

* test(oauth): verify legacy IDs and auth source isolation

---------

Co-authored-by: Noru Wyrms <wyrmsnoru@gmail.com>

internal/model/users_test.go

@@ -0,0 +1,31 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package model
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+// TestOAuthUserInfoIgnoresProviderID verifies nonstandard IDs cannot affect OIDC identity parsing.
+func TestOAuthUserInfoIgnoresProviderID(t *testing.T) {
+	for _, providerID := range []string{`"a27dfc56-07ae-4c9d-9e5c-99103bd8805f"`, `"88888"`, `88888`, `null`} {
+		t.Run(providerID, func(t *testing.T) {
+			payload := `{"id":` + providerID + `,"sub":"external-subject","preferred_username":"oidc-user","email":"oidc@example.com","name":"OIDC User"}`
+			var info OAuthUserInfo
+			if err := json.Unmarshal([]byte(payload), &info); err != nil {
+				t.Fatalf("json.Unmarshal(OAuthUserInfo, id=%s) error = %v, want nil", providerID, err)
+			}
+			want := OAuthUserInfo{
+				Sub:               "external-subject",
+				PreferredUsername: "oidc-user",
+				Email:             "oidc@example.com",
+				Name:              "OIDC User",
+			}
+			if info != want {
+				t.Errorf("json.Unmarshal(OAuthUserInfo, id=%s) = %+v, want %+v", providerID, info, want)
+			}
+		})
+	}
+}