Migrate outbound HTTP calls from Guzzle to the Http facade

Replaces every raw GuzzleHttp\Client usage (Nominatim, web.archive.org,
webmention fetch/discover/send, Bridgy syndication, IndieAuth client_id
lookup, contact avatar/h-card fetch, profile image download, and the
CloudConvert screenshot pipeline) with Illuminate\Support\Facades\Http,
and enables Http::preventStrayRequests() globally in tests so any
un-faked outbound call now fails loudly instead of silently hitting the
network.

The CloudConvert retry-until-finished middleware in AppServiceProvider
is replaced by a plain polling loop in SaveScreenshot, which also fixes
a latent bug where the old middleware decoded a response object instead
of its body. Bridgy syndication jobs keep their tries=1/no-retry
semantics unchanged to avoid duplicate publishes. Guzzle's PSR-7 helpers
(Header, UriResolver, Utils) stay in SendWebMentions since Http has no
equivalent for them.

Every affected test file's Guzzle MockHandler/HandlerStack boilerplate
is replaced with Http::fake()/Http::sequence().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P8j7cJhCiYDsGUgB7obKNQ

tests/TestCase.php

@@ -3,11 +3,19 @@
 namespace Tests;
 
 use Illuminate\Foundation\Testing\TestCase as BaseTestCase;
+use Illuminate\Support\Facades\Http;
 
 abstract class TestCase extends BaseTestCase
 {
     use CreatesApplication;
 
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        Http::preventStrayRequests();
+    }
+
     public function removeDirIfEmpty(string $dir): void
     {
         // scandir() will always return `.` and `..` so even an “empty”