Add tier feature design spec, annual billing, fuel type normalization, and admin subscription management

- Add comprehensive tier feature matrix spec defining Free/Basic/Plus/Pro capabilities across recommendations, predictions, history, logs, tools, and family sharing
- Add `stripe_price_id_annual` column to plans table and rename existing column to `stripe_price_id_monthly`
- Normalize legacy fuel type aliases (petrol→e10, diesel→b7_standard) in users table
- Implement BillingController with checkout, portal, success/cancel routes supporting monthly/annual cadence
- Add admin subscription assignment in Filament user edit page with admin-granted subscription support
- Add DowngradeUserOnSubscriptionDeleted listener to disable WhatsApp/SMS preferences on subscription cancellation
- Add MissedNotificationsOverview widget to Filament user detail page
- Add PollFuelPricesTest covering auto-refresh scenarios
- Add PriceReliability enum with reliability classification based on price age
- Add fuelTypes.js constants file exporting FUEL_TYPES from window global

app/Http/Controllers/BillingController.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Enums\PlanTier;
+use Illuminate\Http\RedirectResponse;
+use Illuminate\Http\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+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 $cadence): Response|RedirectResponse
+    {
+        abort_unless(in_array($tier, [PlanTier::Basic->value, PlanTier::Plus->value, PlanTier::Pro->value], true), 404);
+        abort_unless(in_array($cadence, ['monthly', 'annual'], true), 404);
+
+        $priceId = config("services.stripe.prices.{$tier}.{$cadence}");
+
+        abort_if(empty($priceId), 404, "No Stripe price configured for {$tier} {$cadence}");
+
+        return $request->user()
+            ->newSubscription('default', $priceId)
+            ->allowPromotionCodes()
+            ->checkout([
+                'success_url' => route('billing.success').'?session_id={CHECKOUT_SESSION_ID}',
+                'cancel_url' => route('billing.cancel'),
+            ]);
+    }
+
+    /** Redirect the user to the Stripe-hosted Customer Billing Portal. */
+    public function portal(Request $request): Response|RedirectResponse
+    {
+        return $request->user()->redirectToBillingPortal(route('dashboard'));
+    }
+
+    public function success(): RedirectResponse
+    {
+        return redirect()->route('dashboard')->with('status', 'subscription_started');
+    }
+
+    public function cancel(): RedirectResponse
+    {
+        return redirect()->route('dashboard')->with('status', 'subscription_cancelled');
+    }
+}