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

backend/app/Comment/CommentModel.php

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+<?php
+
+namespace App\Comment;
+
+use DateTimeImmutable;
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+
+/**
+ * @property int $id
+ * @property int $post_id
+ * @property int $user_id
+ * @property string $body
+ * @property DateTimeImmutable $created_at
+ *
+ * @method static Builder<static>|CommentModel newModelQuery()
+ * @method static Builder<static>|CommentModel newQuery()
+ * @method static Builder<static>|CommentModel query()
+ * @method static Builder<static>|CommentModel whereId($value)
+ * @method static Builder<static>|CommentModel wherePostId($value)
+ * @method static Builder<static>|CommentModel whereUserId($value)
+ * @method static Builder<static>|CommentModel whereBody($value)
+ * @method static Builder<static>|CommentModel whereCreatedAt($value)
+ *
+ * @mixin \Eloquent
+ */
+class CommentModel extends Model
+{
+    protected $table = 'comments';
+
+    public $timestamps = false;
+
+    protected $fillable = [
+        'post_id',
+        'user_id',
+        'body',
+        'created_at',
+    ];
+
+    protected $casts = [
+        'created_at' => 'immutable_datetime',
+    ];
+}

backend/app/Comment/EloquentCommentRepository.php

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+<?php
+
+namespace App\Comment;
+
+use DateTimeImmutable;
+use DateTimeZone;
+
+class EloquentCommentRepository implements CommentRepository
+{
+    public function create(CreateCommentDto $dto): Comment
+    {
+        $model = CommentModel::create([
+            'post_id' => $dto->postId,
+            'user_id' => $dto->userId,
+            'body' => $dto->body,
+            'created_at' => $dto->createdAt,
+        ]);
+
+        return $this->toDomain($model);
+    }
+
+    public function find(int $id): ?Comment
+    {
+        $model = CommentModel::find($id);
+
+        return $model === null ? null : $this->toDomain($model);
+    }
+
+    /**
+     * @return Comment[]
+     */
+    public function findByPostId(int $postId): array
+    {
+        $models = CommentModel::query()
+            ->where('post_id', $postId)
+            ->orderBy('created_at', 'asc')
+            ->get();
+
+        return $models->map(
+            function (CommentModel $model) {
+                return $this->toDomain($model);
+            },
+        )->all();
+    }
+
+    public function delete(int $id): void
+    {
+        CommentModel::query()->where('id', $id)->delete();
+    }
+
+    private function toDomain(CommentModel $model): Comment
+    {
+        $utc = new DateTimeZone('UTC');
+
+        return new Comment(
+            id: $model->id,
+            postId: $model->post_id,
+            userId: $model->user_id,
+            body: $model->body,
+            createdAt: new DateTimeImmutable(
+                $model->created_at->toDateTimeString(),
+                $utc,
+            ),
+        );
+    }
+}

backend/database/migrations/2026_05_06_000004_create_comments_table.php

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+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('comments', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('post_id')
+                ->constrained('posts')
+                ->cascadeOnDelete();
+            $table->foreignId('user_id')
+                ->constrained('users')
+                ->cascadeOnDelete();
+            $table->text('body');
+            $table->dateTime('created_at')->index();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('comments');
+    }
+};

backend/tests/Fakes/FakeCommentRepository.php

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+<?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;
+    }
+}