Update existing draft articles instead of erroring on a repeat Micropub post

If a Micropub h-entry post's title matches an existing article that's
still a draft, update that article in place rather than trying to
insert a duplicate. If it matches one that's already published,
reject the request with a clear error instead of silently colliding.

Also set includeTrashed on Article's slug config as a safety net: this
model soft-deletes, and Sluggable's uniqueness check ignores trashed
rows by default, so a previously-deleted article's title could crash
new inserts with a raw unique constraint violation (this is exactly
what surfaced in Flare as a UniqueConstraintViolationException on
articles_titleurl_unique once the prior swallowed-exception fix
shipped).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/Unit/ArticlesTest.php

@@ -74,6 +74,17 @@ class ArticlesTest extends TestCase
         $this->assertEquals(config('app.url').$article->link, $article->uri);
     }
 
+    #[Test]
+    public function slug_is_suffixed_when_a_trashed_article_already_used_it(): void
+    {
+        $original = Article::create(['title' => 'My Title', 'main' => 'Content']);
+        $original->delete();
+
+        $newArticle = Article::create(['title' => 'My Title', 'main' => 'Other content']);
+
+        $this->assertEquals('my-title-2', $newArticle->titleurl);
+    }
+
     #[Test]
     public function date_scope_returns_expected_articles(): void
     {