add explicit set levels

backend/app/Providers/AppServiceProvider.php

@@ -19,7 +19,9 @@ use App\Email\LaravelEmailFactory;
 use App\Element\ElementRepository;
 use App\Element\EloquentElementRepository;
 use App\Set\EloquentSetRepository;
+use App\Set\EloquentSetLevelRepository;
 use App\Set\SetRepository;
+use App\Set\SetLevelRepository;
 use App\Schedule\EloquentScheduleRepository;
 use App\Schedule\ScheduleRepository;
 use App\User\EloquentUserRepository;
@@ -55,6 +57,10 @@ class AppServiceProvider extends ServiceProvider
             SetRepository::class,
             EloquentSetRepository::class,
         );
+        $this->app->bind(
+            SetLevelRepository::class,
+            EloquentSetLevelRepository::class,
+        );
         $this->app->bind(
             ElementRepository::class,
             EloquentElementRepository::class,

backend/app/Set/CreateSetLevelDto.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace App\Set;
+
+final readonly class CreateSetLevelDto
+{
+    public function __construct(
+        public Set $set,
+        public string $kind,
+    ) {}
+}

backend/app/Set/EloquentSetLevelRepository.php

@@ -0,0 +1,88 @@
+<?php
+
+namespace App\Set;
+
+use DomainException;
+use RuntimeException;
+
+class EloquentSetLevelRepository implements SetLevelRepository
+{
+    public function __construct(
+        private SetRepository $setRepository,
+    ) {}
+
+    public function create(CreateSetLevelDto $dto): SetLevel
+    {
+        $kindExists = SetLevelModel::query()
+            ->where('set_id', $dto->set->getId())
+            ->where('kind', $dto->kind)
+            ->exists();
+        if ($kindExists) {
+            throw new DomainException(
+                'level kind must be unique within set',
+            );
+        }
+
+        $currentMaximum = SetLevelModel::query()
+            ->where('set_id', $dto->set->getId())
+            ->max('depth');
+        $depth = $currentMaximum === null
+            ? 0
+            : (int) $currentMaximum + 1;
+        $model = SetLevelModel::create([
+            'set_id' => $dto->set->getId(),
+            'kind' => $dto->kind,
+            'depth' => $depth,
+        ]);
+
+        return new SetLevel(
+            id: $model->id,
+            set: $dto->set,
+            kind: $model->kind,
+            depth: $model->depth,
+        );
+    }
+
+    public function find(int $id): ?SetLevel
+    {
+        $model = SetLevelModel::find($id);
+
+        return $model === null ? null : $this->toDomain($model);
+    }
+
+    public function findBySet(Set $set): array
+    {
+        $models = SetLevelModel::query()
+            ->where('set_id', $set->getId())
+            ->orderBy('depth')
+            ->orderBy('id')
+            ->get();
+        $levels = [];
+
+        foreach ($models as $model) {
+            $levels[] = new SetLevel(
+                id: $model->id,
+                set: $set,
+                kind: $model->kind,
+                depth: $model->depth,
+            );
+        }
+
+        return $levels;
+    }
+
+    private function toDomain(SetLevelModel $model): SetLevel
+    {
+        $set = $this->setRepository->find($model->set_id);
+        if ($set === null) {
+            throw new RuntimeException('set level set not found');
+        }
+
+        return new SetLevel(
+            id: $model->id,
+            set: $set,
+            kind: $model->kind,
+            depth: $model->depth,
+        );
+    }
+}

backend/app/Set/SetLevel.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Set;
+
+final readonly class SetLevel
+{
+    public function __construct(
+        private int $id,
+        private Set $set,
+        private string $kind,
+        private int $depth,
+    ) {}
+
+    public function getId(): int
+    {
+        return $this->id;
+    }
+
+    public function getSet(): Set
+    {
+        return $this->set;
+    }
+
+    public function getKind(): string
+    {
+        return $this->kind;
+    }
+
+    public function getDepth(): int
+    {
+        return $this->depth;
+    }
+}

backend/app/Set/SetLevelModel.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace App\Set;
+
+use Illuminate\Database\Eloquent\Attributes\Fillable;
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+
+/**
+ * @property int $id
+ * @property int $set_id
+ * @property string $kind
+ * @property int $depth
+ *
+ * @method static Builder<static>|SetLevelModel newModelQuery()
+ * @method static Builder<static>|SetLevelModel newQuery()
+ * @method static Builder<static>|SetLevelModel query()
+ *
+ * @mixin \Eloquent
+ */
+#[Fillable(['set_id', 'kind', 'depth'])]
+class SetLevelModel extends Model
+{
+    protected $table = 'set_levels';
+
+    public $timestamps = false;
+
+    /**
+     * @return array<string, string>
+     */
+    protected function casts(): array
+    {
+        return [
+            'set_id' => 'integer',
+            'depth' => 'integer',
+        ];
+    }
+}

backend/app/Set/SetLevelRepository.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Set;
+
+use DomainException;
+
+interface SetLevelRepository
+{
+    /**
+     * @throws DomainException
+     */
+    public function create(CreateSetLevelDto $dto): SetLevel;
+
+    public function find(int $id): ?SetLevel;
+
+    /**
+     * @return list<SetLevel>
+     */
+    public function findBySet(Set $set): array;
+}

backend/database/migrations/2026_08_07_000000_create_set_levels_table.php

@@ -0,0 +1,27 @@
+<?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('set_levels', function (Blueprint $table): void {
+            $table->id();
+            $table->foreignId('set_id')
+                ->constrained('sets')
+                ->cascadeOnDelete();
+            $table->string('kind');
+            $table->unsignedInteger('depth');
+            $table->unique(['set_id', 'kind']);
+            $table->unique(['set_id', 'depth']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('set_levels');
+    }
+};