🏆 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.

database/factories/HighscoreFactory.php

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+<?php
+
+namespace Database\Factories;
+
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+/**
+ * @extends \Illuminate\Database\Eloquent\Factories\Factory<\App\Models\Highscore>
+ */
+class HighscoreFactory extends Factory
+{
+    /**
+     * Define the model's default state.
+     *
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        return [
+            'npub' => 'npub1'.fake()->regexify('[a-z0-9]{20}'),
+            'name' => fake()->name(),
+            'satoshis' => fake()->numberBetween(0, 100000),
+            'blocks' => fake()->numberBetween(0, 1000),
+            'achieved_at' => fake()->dateTimeBetween('-1 year', 'now'),
+        ];
+    }
+}

database/migrations/2026_02_02_105837_create_highscores_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
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('highscores', function (Blueprint $table) {
+            $table->id();
+            $table->string('npub', 100);
+            $table->string('name')->nullable();
+            $table->unsignedBigInteger('satoshis');
+            $table->unsignedInteger('blocks');
+            $table->dateTime('achieved_at');
+            $table->timestamps();
+
+            $table->unique(['npub', 'achieved_at']);
+            $table->index('satoshis');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('highscores');
+    }
+};