Laravel - Model Class not found

Laravel 5 promotes the use of namespaces for things like Models and Controllers. Your Model is under the App namespace, so your code needs to call it like this:

Route::get('/posts', function(){

        $results = AppPost::all();
        return $results;
});

As mentioned in the comments you can also use or import a namespace in to a file so you don't need to quote the full path, like this:

use AppPost;

Route::get('/posts', function(){

        $results = Post::all();
        return $results;
});

While I'm doing a short primer on namespaces I might as well mention the ability to alias a class as well. Doing this means you can essentially rename your class just in the scope of one file, like this:

use AppPost as PostModel;

Route::get('/posts', function(){

        $results = PostModel::all();
        return $results;
});

More info on importing and aliasing namespaces here: http://php.net/manual/en/language.namespaces.importing.php

Make sure to get() the subpages as IlluminateSupportCollection and mapWithKeys() to reformat the results. Use toArray() to provide the format Nova assumes:

private function selectOptions(): array
{
    $subpages = DB::table('subpages')->get();
    return $subpages->mapWithKeys(function ($subpage) {
        return [$subpage->slug => $subpage->slug];
    })->toArray();
}

This is how the returned result should look like:

[
    'my-article-1' => 'my-article-1',
    'my-article-2' => 'my-article-2',
    'my-article-3' => 'my-article-3',
]

Anytime you create a new class file in L4 run this command.

php composer dump-autoload