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>

tests/Feature/Livewire/SocialAccountsTest.php

@@ -2,12 +2,14 @@
 
 use App\Livewire\SocialAccounts\Index;
 use App\Models\SocialAccount;
+use App\Models\User;
 use Livewire\Livewire;
 
 test('social accounts index page lists existing accounts', function () {
     SocialAccount::factory()->create(['username' => 'example_user']);
 
-    $this->get(route('social-accounts.index'))
+    $this->actingAs(User::factory()->create())
+        ->get(route('social-accounts.index'))
         ->assertOk()
         ->assertSee('example_user');
 });