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>

tests/Feature/PostModelTest.php

@@ -0,0 +1,85 @@
+<?php
+
+use App\Models\Post;
+use App\Models\Site;
+use App\Models\SocialAccount;
+
+test('a post can be created with required fields only', function () {
+    $site = Site::factory()->create();
+
+    $post = Post::create([
+        'site_id' => $site->id,
+        'type' => 'blog',
+        'body' => 'Hello world',
+    ]);
+
+    $fresh = $post->fresh();
+
+    expect($fresh)
+        ->site_id->toBe($site->id)
+        ->type->toBe('blog')
+        ->body->toBe('Hello world')
+        ->status->toBe('draft')
+        ->social_account_id->toBeNull()
+        ->title->toBeNull()
+        ->scheduled_at->toBeNull();
+});
+
+test('a post can be created with an account, title, status, and schedule', function () {
+    $site = Site::factory()->create();
+    $account = SocialAccount::factory()->create();
+
+    $post = Post::create([
+        'site_id' => $site->id,
+        'social_account_id' => $account->id,
+        'type' => 'twitter',
+        'title' => 'Launch day',
+        'body' => 'We just launched!',
+        'status' => 'scheduled',
+        'scheduled_at' => '2026-08-01 09:00:00',
+    ]);
+
+    $fresh = $post->fresh();
+
+    expect($fresh)
+        ->social_account_id->toBe($account->id)
+        ->title->toBe('Launch day')
+        ->status->toBe('scheduled')
+        ->scheduled_at->not->toBeNull();
+});
+
+test('a post belongs to a site', function () {
+    $site = Site::factory()->create();
+    $post = Post::factory()->create(['site_id' => $site->id]);
+
+    expect($post->site)->toBeInstanceOf(Site::class);
+    expect($post->site->id)->toBe($site->id);
+});
+
+test('a post can belong to a social account', function () {
+    $account = SocialAccount::factory()->create();
+    $post = Post::factory()->create(['social_account_id' => $account->id]);
+
+    expect($post->socialAccount)->toBeInstanceOf(SocialAccount::class);
+    expect($post->socialAccount->id)->toBe($account->id);
+});
+
+test('deleting a site cascades and deletes its posts', function () {
+    $site = Site::factory()->create();
+    $post = Post::factory()->create(['site_id' => $site->id]);
+
+    $site->delete();
+
+    expect(Post::find($post->id))->toBeNull();
+});
+
+test('deleting a social account nulls out the post link but keeps the post', function () {
+    $account = SocialAccount::factory()->create();
+    $post = Post::factory()->create(['social_account_id' => $account->id]);
+
+    $account->delete();
+
+    expect($post->fresh())
+        ->not->toBeNull()
+        ->social_account_id->toBeNull();
+});