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

backend/tests/Fakes/FakeCommentRepository.php

@@ -0,0 +1,82 @@
+<?php
+
+namespace Tests\Fakes;
+
+use App\Comment\Comment;
+use App\Comment\CommentRepository;
+use App\Comment\CreateCommentDto;
+
+class FakeCommentRepository implements CommentRepository
+{
+    /**
+     * @var Comment[]
+     */
+    private array $existingComments = [];
+
+    public function create(CreateCommentDto $dto): Comment
+    {
+        $id = $this->getNextId();
+        $comment = new Comment(
+            id: $id,
+            postId: $dto->postId,
+            userId: $dto->userId,
+            body: $dto->body,
+            createdAt: $dto->createdAt,
+        );
+        $this->existingComments[$id] = $comment;
+
+        return $this->copy($comment);
+    }
+
+    public function find(int $id): ?Comment
+    {
+        $comment = $this->existingComments[$id] ?? null;
+        if ($comment === null) {
+            return null;
+        }
+
+        return $this->copy($comment);
+    }
+
+    /**
+     * @return Comment[]
+     */
+    public function findByPostId(int $postId): array
+    {
+        $matching = [];
+        foreach ($this->existingComments as $comment) {
+            if ($comment->getPostId() === $postId) {
+                $matching[] = $this->copy($comment);
+            }
+        }
+        usort(
+            $matching,
+            function (Comment $left, Comment $right) {
+                return $left->getCreatedAt() <=> $right->getCreatedAt();
+            },
+        );
+
+        return $matching;
+    }
+
+    public function delete(int $id): void
+    {
+        unset($this->existingComments[$id]);
+    }
+
+    private function copy(Comment $comment): Comment
+    {
+        return new Comment(
+            id: $comment->getId(),
+            postId: $comment->getPostId(),
+            userId: $comment->getUserId(),
+            body: $comment->getBody(),
+            createdAt: $comment->getCreatedAt(),
+        );
+    }
+
+    private function getNextId(): int
+    {
+        return count($this->existingComments) + 1;
+    }
+}