fix(sec): consider webauthn for external login

- Currently during external login (such as OAuth2), if the user is
enrolled into Webauthn and not enrolled into TOTP then no 2FA is being
done during external login and when account linking is set to `auto` then
also during automatic linking. This results in bypassing the 2FA of the
user.
- Create a new unified function that checks if the user is enrolled into
2FA and use this when necessary. Rename the old `HasTwoFactorByUID`
function to `HasTOTPByUID` which is a more appropiate naming.

(cherry picked from commit df5d656827)

Conflicts:
  the original commit was trimmed down to be fit for backport

models/auth/two_factor_test.go

@@ -0,0 +1,35 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+package auth
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHasTwoFactorByUID(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+
+	t.Run("No twofactor", func(t *testing.T) {
+		ok, err := HasTwoFactorByUID(db.DefaultContext, 2)
+		require.NoError(t, err)
+		assert.False(t, ok)
+	})
+
+	t.Run("WebAuthn credential", func(t *testing.T) {
+		ok, err := HasTwoFactorByUID(db.DefaultContext, 32)
+		require.NoError(t, err)
+		assert.True(t, ok)
+	})
+
+	t.Run("TOTP", func(t *testing.T) {
+		ok, err := HasTwoFactorByUID(db.DefaultContext, 24)
+		require.NoError(t, err)
+		assert.True(t, ok)
+	})
+}