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/tests/Fakes/FakePostRepository.php

@@ -0,0 +1,103 @@
+<?php
+
+namespace Tests\Fakes;
+
+use App\Post\CreatePostDto;
+use App\Post\Post;
+use App\Post\PostRepository;
+
+class FakePostRepository implements PostRepository
+{
+    /**
+     * @var Post[]
+     */
+    private array $existingPosts = [];
+
+    public function create(CreatePostDto $dto): Post
+    {
+        $id = $this->getNextId();
+        $post = new Post(
+            id: $id,
+            userId: $dto->userId,
+            title: $dto->title,
+            body: $dto->body,
+            createdAt: $dto->createdAt,
+        );
+        $this->existingPosts[$id] = $post;
+
+        return $post;
+    }
+
+    public function find(int $id): ?Post
+    {
+        $post = $this->existingPosts[$id] ?? null;
+        if ($post === null) {
+            return null;
+        }
+
+        return $this->copy($post);
+    }
+
+    /**
+     * @return Post[]
+     */
+    public function findByUserId(int $userId): array
+    {
+        $matching = [];
+        foreach ($this->existingPosts as $post) {
+            if ($post->getUserId() === $userId) {
+                $matching[] = $this->copy($post);
+            }
+        }
+        usort(
+            $matching,
+            function (Post $left, Post $right) {
+                return $right->getCreatedAt() <=> $left->getCreatedAt();
+            },
+        );
+
+        return $matching;
+    }
+
+    /**
+     * @return Post[]
+     */
+    public function findRecent(int $limit): array
+    {
+        $all = array_map(
+            function (Post $post) {
+                return $this->copy($post);
+            },
+            array_values($this->existingPosts),
+        );
+        usort(
+            $all,
+            function (Post $left, Post $right) {
+                return $right->getCreatedAt() <=> $left->getCreatedAt();
+            },
+        );
+
+        return array_slice($all, 0, $limit);
+    }
+
+    public function delete(int $id): void
+    {
+        unset($this->existingPosts[$id]);
+    }
+
+    private function copy(Post $post): Post
+    {
+        return new Post(
+            id: $post->getId(),
+            userId: $post->getUserId(),
+            title: $post->getTitle(),
+            body: $post->getBody(),
+            createdAt: $post->getCreatedAt(),
+        );
+    }
+
+    private function getNextId(): int
+    {
+        return count($this->existingPosts) + 1;
+    }
+}