🏆 Add highscore feature with API endpoints, validations, and tests

- **Added:** Endpoints for submitting highscores (`highscores.store`) and retrieving the leaderboard (`highscores.index`).
- **Implemented:** Validation rules via `StoreHighscoreRequest` to ensure highscore integrity.
- **Included:** `Highscore` model, migration, and factory for data handling and seeding.
- **Enhanced:** Comprehensive feature tests covering submission, updating, retrieval, and payload validation.

app/Models/Highscore.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+class Highscore extends Model
+{
+    use HasFactory;
+
+    /**
+     * The attributes that aren't mass assignable.
+     *
+     * @var array
+     */
+    protected $guarded = [];
+
+    /**
+     * The attributes that should be cast to native types.
+     *
+     * @var array
+     */
+    protected $casts = [
+        'id' => 'integer',
+        'satoshis' => 'integer',
+        'blocks' => 'integer',
+        'achieved_at' => 'datetime',
+    ];
+}