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>

app/Models/Post.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class Post extends Model
+{
+    use HasFactory;
+
+    public const TYPES = [
+        'twitter', 'instagram', 'facebook', 'reddit',
+        'tiktok', 'linkedin', 'youtube', 'blog', 'other',
+    ];
+
+    public const STATUSES = ['draft', 'scheduled', 'published', 'failed'];
+
+    protected $fillable = [
+        'site_id',
+        'social_account_id',
+        'type',
+        'title',
+        'body',
+        'status',
+        'scheduled_at',
+    ];
+
+    protected $casts = [
+        'scheduled_at' => 'datetime',
+    ];
+
+    public function site(): BelongsTo
+    {
+        return $this->belongsTo(Site::class);
+    }
+
+    public function socialAccount(): BelongsTo
+    {
+        return $this->belongsTo(SocialAccount::class);
+    }
+}