Add a config option to block "expensive" pages for anonymous users (#34024)

Fix #33966

```
;; User must sign in to view anything.
;; It could be set to "expensive" to block anonymous users accessing some pages which consume a lot of resources,
;; for example: block anonymous AI crawlers from accessing repo code pages.
;; The "expensive" mode is experimental and subject to change.
;REQUIRE_SIGNIN_VIEW = false
```

routers/common/blockexpensive_test.go

@@ -0,0 +1,30 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBlockExpensive(t *testing.T) {
+	cases := []struct {
+		expensive bool
+		routePath string
+	}{
+		{false, "/user/xxx"},
+		{false, "/login/xxx"},
+		{true, "/{username}/{reponame}/archive/xxx"},
+		{true, "/{username}/{reponame}/graph"},
+		{true, "/{username}/{reponame}/src/xxx"},
+		{true, "/{username}/{reponame}/wiki/xxx"},
+		{true, "/{username}/{reponame}/activity/xxx"},
+	}
+	for _, c := range cases {
+		assert.Equal(t, c.expensive, isRoutePathExpensive(c.routePath), "routePath: %s", c.routePath)
+	}
+
+	assert.True(t, isRoutePathForLongPolling("/user/events"))
+}