feat: add published_url column to posts

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

tests/Feature/PostModelTest.php

@@ -83,3 +83,17 @@ test('deleting a social account nulls out the post link but keeps the post', fun
         ->not->toBeNull()
         ->social_account_id->toBeNull();
 });
+
+test('a post can store a published_url', function () {
+    $post = Post::factory()->create(['published_url' => null]);
+
+    $post->update(['published_url' => 'https://example.com/2026/07/my-post']);
+
+    expect($post->fresh()->published_url)->toBe('https://example.com/2026/07/my-post');
+});
+
+test('published_url defaults to null', function () {
+    $post = Post::factory()->create();
+
+    expect($post->fresh()->published_url)->toBeNull();
+});