feat(payments): implement standard webhooks validation system

Add comprehensive webhook validation and processing system with Polar.sh integration:

  - Create built-in Standard Webhooks package following official specification
  - Implement HMAC-SHA256 signature validation with base64 encoding
  - Add webhook factory for multi-provider support (Polar, Stripe, generic)
  - Replace custom Polar webhook validation with Standard Webhooks implementation
  - Add proper exception handling with custom WebhookVerificationException
  - Support sandbox mode bypass for development environments
  - Update Polar provider to use database-driven configuration
  - Enhance webhook test suite with proper Standard Webhooks format
  - Add PaymentProvider model HasFactory trait for testing
  - Implement timestamp tolerance checking (±5 minutes) for replay protection
  - Support multiple signature versions and proper header validation

  This provides a secure, reusable webhook validation system that can be extended
  to other payment providers while maintaining full compliance with Standard
  Webhooks specification.

  BREAKING CHANGE: Polar webhook validation now uses Standard Webhooks format
  with headers 'webhook-id', 'webhook-timestamp', 'webhook-signature' instead of
  previous Polar-specific headers.

app/Models/PaymentProvider.php

@@ -2,10 +2,13 @@
 
 namespace App\Models;
 
+use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 
 class PaymentProvider extends Model
 {
+    use HasFactory;
+
     protected $fillable = [
         'name',
         'display_name',

app/Models/Subscription.php

@@ -3,11 +3,14 @@
 namespace App\Models;
 
 use App\Services\Payments\PaymentOrchestrator;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Support\Facades\Log;
 
 class Subscription extends Model
 {
+    use HasFactory;
+
     protected $fillable = [
         'user_id',
         'type',
@@ -49,6 +52,16 @@ class Subscription extends Model
         'order_created_at',
         'order_paid_at',
         'provider_checkout_id',
+        'resume_reason',
+        'trial_ended_at',
+        'trial_converted_to',
+        'subscription_id_fetched_at',
+        'polar_dates',
+        'customer_state_changed_at',
+        'polar_subscription_data',
+        'customer_metadata',
+        'trial_will_end_sent_at',
+        'pause_reason',
     ];
 
     protected $casts = [