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

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+<?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'),
+        ];
+    }
+}

database/migrations/2026_07_18_135721_create_posts_table.php

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+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('posts', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('site_id')->constrained()->cascadeOnDelete();
+            $table->foreignId('social_account_id')->nullable()->constrained()->nullOnDelete();
+            $table->enum('type', [
+                'twitter', 'instagram', 'facebook', 'reddit',
+                'tiktok', 'linkedin', 'youtube', 'blog', 'other',
+            ]);
+            $table->string('title')->nullable();
+            $table->text('body');
+            $table->enum('status', ['draft', 'scheduled', 'published', 'failed'])->default('draft');
+            $table->timestamp('scheduled_at')->nullable();
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('posts');
+    }
+};