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/Controllers/Api/PostController.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace App\Http\Controllers\Api;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\StorePostRequest;
+use App\Http\Requests\UpdatePostRequest;
+use App\Http\Resources\PostResource;
+use App\Models\Post;
+use Illuminate\Http\Request;
+
+class PostController extends Controller
+{
+    public function index(Request $request)
+    {
+        $query = Post::query();
+
+        if ($request->filled('status')) {
+            $query->where('status', $request->string('status'));
+        }
+
+        if ($request->filled('site_id')) {
+            $query->where('site_id', $request->integer('site_id'));
+        }
+
+        if ($request->filled('social_account_id')) {
+            $query->where('social_account_id', $request->integer('social_account_id'));
+        }
+
+        if ($request->boolean('due')) {
+            $query->where('status', 'scheduled')
+                ->where('scheduled_at', '<=', now());
+        }
+
+        return PostResource::collection($query->orderByDesc('created_at')->get());
+    }
+
+    public function show(Post $post)
+    {
+        return new PostResource($post);
+    }
+
+    public function store(StorePostRequest $request)
+    {
+        $post = Post::create($request->validated())->fresh();
+
+        return (new PostResource($post))->response()->setStatusCode(201);
+    }
+
+    public function update(UpdatePostRequest $request, Post $post)
+    {
+        $post->update($request->validated());
+
+        return new PostResource($post);
+    }
+
+    public function destroy(Post $post)
+    {
+        $post->delete();
+
+        return response()->noContent();
+    }
+}