Add a Publisher abstraction and queued job to publish from the UI

Introduce Publisher/PublisherManager (WordPress + Reddit implementations)
so publishing logic is shared between the CLI, the queue, and future
network integrations. Add a "Publish" button on the Posts page that
dispatches PublishPostJob instead of blocking the request — Reddit's
publisher shells out to the existing Playwright sidecar and can take
tens of seconds, so it can't run synchronously in a web request.

Adds a `target` field (e.g. subreddit) and `publishing` status to Post,
plus a `publish_error` field so failures surface in the UI. Refactors
posts:publish to delegate to WordPressPublisher without changing its
CLI output contract (existing tests pass unchanged).

Also fixes two bugs found along the way: UpdatePostRequest was silently
dropping `published_url` from PATCH requests (never in its validation
rules, despite AI_API.md documenting it), and three Livewire index-page
tests were broken by the earlier auth change (never re-run after adding
the `auth` middleware) since they hit routes via HTTP without a logged-in
user.

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

app/Http/Requests/UpdatePostRequest.php

@@ -20,8 +20,10 @@ class UpdatePostRequest extends FormRequest
             'type' => ['sometimes', 'required', 'string', 'in:' . implode(',', Post::TYPES)],
             'title' => ['nullable', 'string', 'max:255'],
             'body' => ['sometimes', 'required', 'string'],
+            'target' => ['nullable', 'string', 'max:255'],
             'status' => ['sometimes', 'string', 'in:' . implode(',', Post::STATUSES)],
             'scheduled_at' => ['nullable', 'date'],
+            'published_url' => ['nullable', 'string', 'max:255'],
         ];
     }
 }