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>

app/Models/Article.php

@@ -40,6 +40,7 @@ class Article extends Model
         return [
             'titleurl' => [
                 'source' => 'title',
+                'includeTrashed' => true,
             ],
         ];
     }