add Post persistence: model, migration, eloquent + fake repo

PostModel maps posts table (id, user_id fk, title, body text,
created_at indexed). EloquentPostRepository: create, find,
findByUserId (desc by created_at), findRecent (limit, desc),
delete - chain via ::query() to keep larastan happy.
FakePostRepository sorts on read (defensive copy each return).
cascade-on-delete on user_id so removing a user nukes their
posts.

phpstan.neon suppresses staticMethod.dynamicCall under
app/*/Eloquent*Repository.php - phpstan-strict-rules flags
Eloquent's fluent builder idiom (Model::query()->orderBy())
because the static methods become instance calls mid-chain.
suppression scoped to repo files only so the rule still
applies elsewhere.

backend/app/Post/PostModel.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace App\Post;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Support\Carbon;
+
+/**
+ * @property int $id
+ * @property int $user_id
+ * @property string $title
+ * @property string $body
+ * @property Carbon $created_at
+ *
+ * @method static Builder<static>|PostModel newModelQuery()
+ * @method static Builder<static>|PostModel newQuery()
+ * @method static Builder<static>|PostModel query()
+ * @method static Builder<static>|PostModel whereId($value)
+ * @method static Builder<static>|PostModel whereUserId($value)
+ *
+ * @mixin \Eloquent
+ */
+class PostModel extends Model
+{
+    protected $table = 'posts';
+
+    public $timestamps = false;
+
+    protected $fillable = [
+        'user_id',
+        'title',
+        'body',
+        'created_at',
+    ];
+
+    protected $casts = [
+        'created_at' => 'datetime',
+    ];
+}