Add middleware for request prioritization (#33951)

This adds a middleware for overload protection that is intended to help protect against malicious scrapers.
It does this via [`codel`](https://github.com/bohde/codel), which will perform the following:

1. Limit the number of in-flight requests to some user-defined max
2. When in-flight requests have reached their begin queuing requests.
    Logged-in requests having priority above logged-out requests
3. Once a request has been queued for too long,
    it has a probabilistic chance to be rejected based on how overloaded the entire system is.

When a server experiences more traffic than it can handle,
this keeps latency low for logged-in users and rejects just
enough requests from logged-out users to not overload the service.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

routers/common/qos_test.go

@@ -0,0 +1,91 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"net/http"
+	"testing"
+
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/web/middleware"
+	"code.gitea.io/gitea/services/contexttest"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRequestPriority(t *testing.T) {
+	type test struct {
+		Name         string
+		User         *user_model.User
+		RoutePattern string
+		Expected     Priority
+	}
+
+	cases := []test{
+		{
+			Name:     "Logged In",
+			User:     &user_model.User{},
+			Expected: HighPriority,
+		},
+		{
+			Name:         "Sign In",
+			RoutePattern: "/user/login",
+			Expected:     DefaultPriority,
+		},
+		{
+			Name:         "Repo Home",
+			RoutePattern: "/{username}/{reponame}",
+			Expected:     DefaultPriority,
+		},
+		{
+			Name:         "User Repo",
+			RoutePattern: "/{username}/{reponame}/src/branch/main",
+			Expected:     LowPriority,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.Name, func(t *testing.T) {
+			ctx, _ := contexttest.MockContext(t, "")
+
+			if tc.User != nil {
+				data := middleware.GetContextData(ctx)
+				data[middleware.ContextDataKeySignedUser] = tc.User
+			}
+
+			rctx := chi.RouteContext(ctx)
+			rctx.RoutePatterns = []string{tc.RoutePattern}
+
+			assert.Exactly(t, tc.Expected, requestPriority(ctx))
+		})
+	}
+}
+
+func TestRenderServiceUnavailable(t *testing.T) {
+	t.Run("HTML", func(t *testing.T) {
+		ctx, resp := contexttest.MockContext(t, "")
+		ctx.Req.Header.Set("Accept", "text/html")
+
+		renderServiceUnavailable(resp, ctx.Req)
+		assert.Equal(t, http.StatusServiceUnavailable, resp.Code)
+		assert.Contains(t, resp.Header().Get("Content-Type"), "text/html")
+
+		body := resp.Body.String()
+		assert.Contains(t, body, `lang="en-US"`)
+		assert.Contains(t, body, "503 Service Unavailable")
+	})
+
+	t.Run("plain", func(t *testing.T) {
+		ctx, resp := contexttest.MockContext(t, "")
+		ctx.Req.Header.Set("Accept", "text/plain")
+
+		renderServiceUnavailable(resp, ctx.Req)
+		assert.Equal(t, http.StatusServiceUnavailable, resp.Code)
+		assert.Contains(t, resp.Header().Get("Content-Type"), "text/plain")
+
+		body := resp.Body.String()
+		assert.Contains(t, body, "503 Service Unavailable")
+	})
+}