Refactor internal routers (partial backport, auth token const time comparing) (#32473) (#32479)

Partially backport #32473. LFS related changes are not in 1.22, so skip
them.

1. Ignore non-existing repos during migrations
2. Improve ReadBatchLine's comment
3. Use `X-Gitea-Internal-Auth` header for internal API calls and make
the comparing constant time (it wasn't a serous problem because in a
real world it's nearly impossible to timing-attack the token, but indeed
security related and good to fix and backport)
4. Fix route mock nil check

modules/web/route.go

@@ -5,6 +5,7 @@ package web
 
 import (
 	"net/http"
+	"reflect"
 	"strings"
 
 	"code.gitea.io/gitea/modules/web/middleware"
@@ -80,15 +81,23 @@ func (r *Route) getPattern(pattern string) string {
 	return strings.TrimSuffix(newPattern, "/")
 }
 
+func isNilOrFuncNil(v any) bool {
+	if v == nil {
+		return true
+	}
+	r := reflect.ValueOf(v)
+	return r.Kind() == reflect.Func && r.IsNil()
+}
+
 func (r *Route) wrapMiddlewareAndHandler(h []any) ([]func(http.Handler) http.Handler, http.HandlerFunc) {
 	handlerProviders := make([]func(http.Handler) http.Handler, 0, len(r.curMiddlewares)+len(h)+1)
 	for _, m := range r.curMiddlewares {
-		if m != nil {
+		if !isNilOrFuncNil(m) {
 			handlerProviders = append(handlerProviders, toHandlerProvider(m))
 		}
 	}
 	for _, m := range h {
-		if h != nil {
+		if !isNilOrFuncNil(m) {
 			handlerProviders = append(handlerProviders, toHandlerProvider(m))
 		}
 	}