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>

docs/features/subscription-billing.md

@@ -34,7 +34,12 @@ All subscription models are defined in `models/database/subscription.py`:
 | `BillingService` | `app/services/billing_service.py` | Subscription operations, checkout, portal |
 | `SubscriptionService` | `app/services/subscription_service.py` | Limit checks, usage tracking |
 | `StripeService` | `app/services/stripe_service.py` | Core Stripe API operations |
-| `StripeWebhookHandler` | `app/services/stripe_webhook_handler.py` | Webhook event processing |
+
+### Handlers
+
+| Handler | Location | Purpose |
+|---------|----------|---------|
+| `StripeWebhookHandler` | `app/handlers/stripe_webhook.py` | Webhook event processing |
 
 ### API Endpoints