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/EloquentPostRepository.php

@@ -0,0 +1,83 @@
+<?php
+
+namespace App\Post;
+
+use DateTimeImmutable;
+use DateTimeZone;
+
+class EloquentPostRepository implements PostRepository
+{
+    public function create(CreatePostDto $dto): Post
+    {
+        $model = PostModel::create([
+            'user_id' => $dto->userId,
+            'title' => $dto->title,
+            'body' => $dto->body,
+            'created_at' => $dto->createdAt,
+        ]);
+
+        return $this->toDomain($model);
+    }
+
+    public function find(int $id): ?Post
+    {
+        $model = PostModel::find($id);
+
+        return $model === null ? null : $this->toDomain($model);
+    }
+
+    /**
+     * @return Post[]
+     */
+    public function findByUserId(int $userId): array
+    {
+        $models = PostModel::query()
+            ->where('user_id', $userId)
+            ->orderBy('created_at', 'desc')
+            ->get();
+
+        return $models->map(
+            function (PostModel $model) {
+                return $this->toDomain($model);
+            },
+        )->all();
+    }
+
+    /**
+     * @return Post[]
+     */
+    public function findRecent(int $limit): array
+    {
+        $models = PostModel::query()
+            ->orderBy('created_at', 'desc')
+            ->limit($limit)
+            ->get();
+
+        return $models->map(
+            function (PostModel $model) {
+                return $this->toDomain($model);
+            },
+        )->all();
+    }
+
+    public function delete(int $id): void
+    {
+        PostModel::query()->where('id', $id)->delete();
+    }
+
+    private function toDomain(PostModel $model): Post
+    {
+        $utc = new DateTimeZone('UTC');
+
+        return new Post(
+            id: $model->id,
+            userId: $model->user_id,
+            title: $model->title,
+            body: $model->body,
+            createdAt: new DateTimeImmutable(
+                $model->created_at->toDateTimeString(),
+                $utc,
+            ),
+        );
+    }
+}

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',
+    ];
+}