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/api/v1/shared/webhooks.py

@@ -14,7 +14,7 @@ from sqlalchemy.orm import Session
 from app.core.database import get_db
 from app.exceptions import InvalidWebhookSignatureException, WebhookMissingSignatureException
 from app.services.stripe_service import stripe_service
-from app.services.stripe_webhook_handler import stripe_webhook_handler
+from app.handlers.stripe_webhook import stripe_webhook_handler
 
 router = APIRouter(prefix="/webhooks")
 logger = logging.getLogger(__name__)