feat(payments): enhance Polar.sh provider with official API compliance

- Add comprehensive rate limiting (300 req/min) with automatic throttling
  - Implement centralized API request method for consistent authentication
  - Add support for Polar-specific webhook events (order.created, order.paid, subscription.active, customer.state_changed, benefit_grant.created)
  - Update API endpoints to match Polar's official structure (remove /v1 prefix)
  - Add external_id support for reliable customer-user mapping
  - Implement sandbox mode with separate credentials configuration
  - Add discount code support in checkout flow
  - Add credential validation method for API connectivity testing
  - Update webhook signature validation and event handling
  - Enhance error handling and logging throughout provider
  - Add proper metadata structure with user and plan information
  - Update services configuration and environment variables for sandbox support

  BREAKING CHANGE: Updated API endpoint structure and webhook event handling to comply with Polar.sh official API specification.

app/Models/Subscription.php

@@ -40,6 +40,14 @@ class Subscription extends Model
         'migration_date',
         'migration_reason',
         'created_at',
+        'activated_at',
+        'checkout_id',
+        'customer_id',
+        'polar_checkout',
+        'order_id',
+        'polar_order',
+        'order_created_at',
+        'order_paid_at',
     ];
 
     protected $casts = [