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)

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

app/handlers/__init__.py

@@ -0,0 +1,10 @@
+# app/handlers/__init__.py
+"""
+Handlers module.
+
+Contains event handlers for webhooks and other external events.
+"""
+
+from app.handlers.stripe_webhook import stripe_webhook_handler
+
+__all__ = ["stripe_webhook_handler"]