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

tests/Feature/Payments/DowngradeUserOnSubscriptionDeletedTest.php

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+<?php
+
+use App\Listeners\DowngradeUserOnSubscriptionDeleted;
+use App\Models\User;
+use App\Models\UserNotificationPreference;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Laravel\Cashier\Events\WebhookReceived;
+
+uses(RefreshDatabase::class);
+
+it('disables whatsapp and sms preferences when a stripe subscription is deleted', function (): void {
+    $user = User::factory()->create(['stripe_id' => 'cus_test_123']);
+
+    UserNotificationPreference::query()->insert([
+        ['user_id' => $user->id, 'channel' => 'whatsapp', 'fuel_type' => 'E10', 'enabled' => true, 'created_at' => now(), 'updated_at' => now()],
+        ['user_id' => $user->id, 'channel' => 'sms', 'fuel_type' => 'E10', 'enabled' => true, 'created_at' => now(), 'updated_at' => now()],
+        ['user_id' => $user->id, 'channel' => 'email', 'fuel_type' => 'E10', 'enabled' => true, 'created_at' => now(), 'updated_at' => now()],
+    ]);
+
+    (new DowngradeUserOnSubscriptionDeleted)->handle(new WebhookReceived([
+        'type' => 'customer.subscription.deleted',
+        'data' => ['object' => ['customer' => 'cus_test_123']],
+    ]));
+
+    expect(UserNotificationPreference::where('user_id', $user->id)->where('channel', 'whatsapp')->value('enabled'))->toBeFalse();
+    expect(UserNotificationPreference::where('user_id', $user->id)->where('channel', 'sms')->value('enabled'))->toBeFalse();
+    expect(UserNotificationPreference::where('user_id', $user->id)->where('channel', 'email')->value('enabled'))->toBeTrue();
+});
+
+it('ignores non-deletion webhook events', function (): void {
+    $user = User::factory()->create(['stripe_id' => 'cus_test_456']);
+    UserNotificationPreference::query()->insert([
+        ['user_id' => $user->id, 'channel' => 'whatsapp', 'fuel_type' => 'E10', 'enabled' => true, 'created_at' => now(), 'updated_at' => now()],
+    ]);
+
+    (new DowngradeUserOnSubscriptionDeleted)->handle(new WebhookReceived([
+        'type' => 'customer.subscription.updated',
+        'data' => ['object' => ['customer' => 'cus_test_456']],
+    ]));
+
+    expect(UserNotificationPreference::where('user_id', $user->id)->where('channel', 'whatsapp')->value('enabled'))->toBeTrue();
+});