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/Plan.php

@@ -77,33 +77,41 @@ class Plan extends Model
      */
     public static function resolveCadenceForUser(User $user): ?string
     {
-        if (! method_exists($user, 'subscriptions')) {
-            return null;
-        }
+        $cache = Cache::supportsTags() ? Cache::tags(['plans']) : Cache::store();
 
-        $priceId = $user->subscriptions()->active()->value('stripe_price');
+        return $cache->remember(
+            "plan_cadence_for_user_{$user->id}",
+            3600,
+            function () use ($user): ?string {
+                if (! method_exists($user, 'subscriptions')) {
+                    return null;
+                }
 
-        if ($priceId === null) {
-            return null;
-        }
+                $priceId = $user->subscriptions()->active()->value('stripe_price');
 
-        $plan = static::where('stripe_price_id_monthly', $priceId)
-            ->orWhere('stripe_price_id_annual', $priceId)
-            ->first();
+                if ($priceId === null) {
+                    return null;
+                }
 
-        if ($plan === null) {
-            return null;
-        }
+                $plan = static::where('stripe_price_id_monthly', $priceId)
+                    ->orWhere('stripe_price_id_annual', $priceId)
+                    ->first();
 
-        if ($plan->stripe_price_id_monthly === $priceId) {
-            return 'monthly';
-        }
+                if ($plan === null) {
+                    return null;
+                }
 
-        if ($plan->stripe_price_id_annual === $priceId) {
-            return 'annual';
-        }
+                if ($plan->stripe_price_id_monthly === $priceId) {
+                    return 'monthly';
+                }
 
-        return null;
+                if ($plan->stripe_price_id_annual === $priceId) {
+                    return 'annual';
+                }
+
+                return null;
+            }
+        );
     }
 
     protected static function booted(): void