Move webmention parsing and authorship into the app

The jonnybarnes/webmentions-parser package is now archived, and its
guzzlehttp/guzzle ^6|^7 constraint was the only thing holding Guzzle
back from 8. Port the parts we use into App\Services\WebMention and
drop the package.

Parser keeps getMentionType and the checkInReplyTo/LikeOf/RepostOf
checks unchanged. Authorship follows the same authorship algorithm,
with some fixes:

- fetch the author page with Laravel's Http client, so tests can fake
  it, and treat connection failures as AuthorshipParserException too
- keep state in locals so one instance can be reused safely
- make the url == uid == author-page check actually compare the
  h-card's uid, rather than an undefined variable

Add direct tests for both, and mark the library as archived on the
projects page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/Exceptions/InvalidMentionException.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions;
+
+use Exception;
+
+class InvalidMentionException extends Exception {}