fix(auth): record last sign-in on reverse proxy login (#38672)

Reverse proxy and SSPI logins establish a session but never recorded
`last_login_unix`, so those users stayed "Never Signed-In" in admin.

The write is folded into the language update that `handleSignIn` already
does, so it stays at one query and only runs when a session is
established.

Fixes https://github.com/go-gitea/gitea/issues/7836

---------

Co-authored-by: roman s <roman.sukach@dust-labs.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/auth/reverseproxy_test.go

@@ -0,0 +1,41 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"testing"
+
+	"gitea.dev/models/unittest"
+	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/session"
+	"gitea.dev/modules/setting"
+	"gitea.dev/modules/test"
+	"gitea.dev/services/contexttest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestReverseProxyLastLogin(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+	defer test.MockVariableValue(&setting.ReverseProxyAuthUser, "X-WEBAUTH-USER")()
+
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	require.Zero(t, user.LastLoginUnix)
+
+	ctx, resp := contexttest.MockContext(t, "/", contexttest.MockContextOption{SessionStore: session.NewMockMemStore("reverse-proxy-last-login")})
+	ctx.Req.Header.Set(setting.ReverseProxyAuthUser, user.Name)
+	rp := &ReverseProxy{CreateSession: true}
+
+	_, err := rp.Verify(ctx.Req, resp, ctx, ctx.Session)
+	require.NoError(t, err)
+	assert.NotZero(t, unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: user.ID}).LastLoginUnix)
+
+	user.LastLoginUnix = 1
+	require.NoError(t, user_model.UpdateUserCols(t.Context(), user, "last_login_unix"))
+
+	_, err = rp.Verify(ctx.Req, resp, ctx, ctx.Session)
+	require.NoError(t, err)
+	assert.EqualValues(t, 1, unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: user.ID}).LastLoginUnix) // no write without a new session
+}