feat: FuelPriceService with OAuth token caching

Also extend Pest TestCase to Unit tests and guard MySQL-only migration
DDL (composite PK + PARTITION BY) behind a driver check so in-memory
SQLite tests can run migrations cleanly.

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

tests/Unit/Services/FuelPriceServiceTest.php

@@ -0,0 +1,40 @@
+<?php
+
+use App\Services\FuelPriceService;
+use App\Services\StationTaggingService;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Illuminate\Support\Facades\Cache;
+use Illuminate\Support\Facades\Http;
+
+uses(RefreshDatabase::class);
+
+beforeEach(function (): void {
+    $this->service = new FuelPriceService(new StationTaggingService());
+});
+
+it('fetches and caches an access token', function (): void {
+    Http::fake([
+        '*/oauth/generate_access_token' => Http::response([
+            'data' => [
+                'access_token' => 'test-token-abc',
+                'expires_in'   => 3600,
+            ],
+        ]),
+    ]);
+
+    $token = $this->service->getAccessToken();
+
+    expect($token)->toBe('test-token-abc');
+    expect(Cache::get('fuel_finder_access_token'))->toBe('test-token-abc');
+});
+
+it('returns cached token without hitting API', function (): void {
+    Cache::put('fuel_finder_access_token', 'cached-token', 3540);
+
+    Http::fake();
+
+    $token = $this->service->getAccessToken();
+
+    expect($token)->toBe('cached-token');
+    Http::assertNothingSent();
+});