feat: add Post model, migration, and factory

Adds the Post model belonging to Site (required, cascade-deletes with
the site) and SocialAccount (nullable, nulled out on account delete).
Foundation for the Scheduled Posts feature; later tasks build the
Livewire CRUD UI and API on top of this model.

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

database/factories/PostFactory.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Database\Factories;
+
+use App\Models\Post;
+use App\Models\Site;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+class PostFactory extends Factory
+{
+    protected $model = Post::class;
+
+    public function definition(): array
+    {
+        return [
+            'site_id' => Site::factory(),
+            'social_account_id' => null,
+            'type' => fake()->randomElement(Post::TYPES),
+            'title' => fake()->optional()->sentence(),
+            'body' => fake()->paragraph(),
+            'status' => fake()->randomElement(Post::STATUSES),
+            'scheduled_at' => fake()->optional()->dateTimeBetween('now', '+1 month'),
+        ];
+    }
+}