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/Requests/UpdatePostRequest.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Http\Requests;
+
+use App\Models\Post;
+use Illuminate\Foundation\Http\FormRequest;
+
+class UpdatePostRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'site_id' => ['sometimes', 'required', 'integer', 'exists:sites,id'],
+            'social_account_id' => ['nullable', 'integer', 'exists:social_accounts,id'],
+            'type' => ['sometimes', 'required', 'string', 'in:' . implode(',', Post::TYPES)],
+            'title' => ['nullable', 'string', 'max:255'],
+            'body' => ['sometimes', 'required', 'string'],
+            'status' => ['sometimes', 'string', 'in:' . implode(',', Post::STATUSES)],
+            'scheduled_at' => ['nullable', 'date'],
+        ];
+    }
+}