feat: add full CRUD Posts API

Adds JSON API for Posts (index/show/store/update/destroy) so an AI
agent can draft posts, read what's due for publishing, and report
back publish/fail status. Unlike the existing read-only Sites/Accounts
APIs, this is full CRUD since the app itself never publishes anything.

- PostController with status/site_id/social_account_id/due filters
- StorePostRequest/UpdatePostRequest validate against Post::TYPES/STATUSES
- PostResource shapes the JSON response
- Route::apiResource('posts', ...) registered with ->names('api.posts')
  to avoid colliding with the existing web posts.index route name
- store() calls ->fresh() after create() so the response reflects the
  DB-level default status ('draft') instead of a stale in-memory null

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

app/Http/Resources/PostResource.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace App\Http\Resources;
+
+use Illuminate\Http\Request;
+use Illuminate\Http\Resources\Json\JsonResource;
+
+class PostResource extends JsonResource
+{
+    public function toArray(Request $request): array
+    {
+        return [
+            'id' => $this->id,
+            'site_id' => $this->site_id,
+            'social_account_id' => $this->social_account_id,
+            'type' => $this->type,
+            'title' => $this->title,
+            'body' => $this->body,
+            'status' => $this->status,
+            'scheduled_at' => $this->scheduled_at,
+            'created_at' => $this->created_at,
+            'updated_at' => $this->updated_at,
+        ];
+    }
+}