Remove prediction API endpoint and integrate into stations search

Consolidate prediction functionality by merging /api/prediction endpoint into /api/stations response. Move prediction logic from PredictionController into StationController, returning prediction data alongside station results. Replace usePrediction composable with unified useStations that returns {stations, meta, prediction}. Remove PredictionRequest, related tests, and unused Vue components (FuelFinderTest, MapTest, RecommendationTest, StationListTest). Add PredictionFull component and UpsellBanner. Extend NationalFuelPredictionService to include weekly_summary (7-day series, yesterday/today averages, cheapest/priciest days) and oil signal from price_predictions table. Update Home.vue to consume prediction from stations response. Add Plan::resolveCadenceForUser helper and configure Cashier to use custom Subscription model.

app/Http/Controllers/BillingController.php

@@ -5,6 +5,7 @@ namespace App\Http\Controllers;
 use App\Enums\PlanTier;
 use Illuminate\Http\RedirectResponse;
 use Illuminate\Http\Request;
+use Laravel\Cashier\Checkout;
 use Symfony\Component\HttpFoundation\Response;
 
 class BillingController extends Controller
@@ -12,7 +13,7 @@ class BillingController extends Controller
     /**
      * Redirect the user to a Stripe Checkout session for the requested plan + cadence.
      */
-    public function checkout(Request $request, string $tier, string $cadence): Response|RedirectResponse
+    public function checkout(Request $request, string $tier, string $cadence): Response|RedirectResponse|Checkout
     {
         abort_unless(in_array($tier, [PlanTier::Basic->value, PlanTier::Plus->value, PlanTier::Pro->value], true), 404);
         abort_unless(in_array($cadence, ['monthly', 'annual'], true), 404);