refactor: define explicit fillable attributes on all Eloquent models

Replace $guarded usage with explicit $fillable arrays across all models.
Sync fillable definitions with current database schema and add tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Models/Subscription.php

@@ -6,7 +6,18 @@ use Illuminate\Database\Eloquent\Model;
 
 class Subscription extends Model
 {
-    protected $guarded = [];
+    protected $fillable = [
+        'stripe_invoice_paid',
+        'stripe_subscription_id',
+        'stripe_customer_id',
+        'stripe_cancel_at_period_end',
+        'stripe_plan_id',
+        'stripe_feedback',
+        'stripe_comment',
+        'stripe_trial_already_ended',
+        'stripe_past_due',
+        'stripe_refunded_at',
+    ];
 
     protected function casts(): array
     {