refactor: move stripe webhook handler to app/handlers/

- Create app/handlers/ directory for event handlers
- Move stripe_webhook_handler.py to app/handlers/stripe_webhook.py
- Update imports in webhooks.py, tests, and docs
- Handlers are distinct from services (event-driven vs request-driven)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/services/test_stripe_webhook_handler.py

@@ -6,7 +6,7 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
-from app.services.stripe_webhook_handler import StripeWebhookHandler
+from app.handlers.stripe_webhook import StripeWebhookHandler
 from models.database.subscription import (
     BillingHistory,
     StripeWebhookEvent,
@@ -59,7 +59,7 @@ class TestStripeWebhookHandlerCheckout:
         """Initialize handler instance before each test."""
         self.handler = StripeWebhookHandler()
 
-    @patch("app.services.stripe_webhook_handler.stripe.Subscription.retrieve")
+    @patch("app.handlers.stripe_webhook.stripe.Subscription.retrieve")
     def test_handle_checkout_completed_success(
         self, mock_stripe_retrieve, db, test_vendor, test_subscription, mock_checkout_event
     ):