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>

routes/api.php

@@ -1,5 +1,6 @@
 <?php
 
+use App\Http\Controllers\Api\PostController;
 use App\Http\Controllers\Api\SiteController;
 use App\Http\Controllers\Api\SocialAccountController;
 use Illuminate\Support\Facades\Route;
@@ -9,3 +10,5 @@ Route::get('/sites/{site}', [SiteController::class, 'show']);
 
 Route::get('/accounts', [SocialAccountController::class, 'index']);
 Route::get('/accounts/{socialAccount}', [SocialAccountController::class, 'show']);
+
+Route::apiResource('posts', PostController::class)->names('api.posts');