fix: model audit cleanups (primaryKey, fuel_type cast, cadence cache)

- StationPriceCurrent: $primaryKey was null; set to 'station_id' + keyType
  string so Eloquent has a sensible default for save() / find() paths.
- UserNotificationPreference: add FuelType enum cast on fuel_type so it
  hydrates as an enum like every other price model.
- Plan::resolveCadenceForUser: cache for 1h under the same plans tag as
  resolveForUser; HandleStripeWebhook busts both keys on subscription
  events.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Models/UserNotificationPreference.php

@@ -2,6 +2,7 @@
 
 namespace App\Models;
 
+use App\Enums\FuelType;
 use Database\Factories\UserNotificationPreferenceFactory;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
@@ -44,6 +45,7 @@ class UserNotificationPreference extends Model
     {
         return [
             'enabled' => 'boolean',
+            'fuel_type' => FuelType::class,
         ];
     }
 }