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/Services/Webhooks/WebhookSigningException.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace App\Services\Webhooks;
+
+use Exception;
+
+class WebhookSigningException extends Exception
+{
+    public function __construct($message, $code = 0, ?\Throwable $previous = null)
+    {
+        parent::__construct($message, $code, $previous);
+    }
+}