feat: add StatsOverviewWidget to admin dashboard

Four-stat overview widget (users, stations, oil prediction, API errors)
with 30s polling registered on the admin panel dashboard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/Feature/Admin/StatsOverviewWidgetTest.php

@@ -0,0 +1,26 @@
+<?php
+
+use App\Filament\Widgets\StatsOverviewWidget;
+use App\Models\ApiLog;
+use App\Models\PricePrediction;
+use App\Models\Station;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Livewire\Livewire;
+
+uses(RefreshDatabase::class);
+
+beforeEach(function () {
+    $this->admin = User::factory()->admin()->create();
+    $this->actingAs($this->admin);
+});
+
+it('renders the stats overview widget', function () {
+    User::factory()->count(3)->create();
+    Station::factory()->count(2)->create();
+    PricePrediction::factory()->create(['generated_at' => now()->subHours(2)]);
+    ApiLog::factory()->count(2)->create(['status_code' => 200, 'error' => null, 'created_at' => now()->subMinutes(30)]);
+
+    Livewire::test(StatsOverviewWidget::class)
+        ->assertOk();
+});