Add Webhook deliveries methods

[evanvolgas]

Summary

The SDK webhooks resource (once added via #10) should include delivery history.

Core API endpoints

• GET /webhooks/deliveries - List webhook delivery attempts
• GET /webhooks/deliveries/{id} - Get delivery details including response

Methods to add

class WebhooksResource:
    def list_deliveries(
        self,
        webhook_id: UUID | None = None,
        status: str | None = None,  # "success" | "failed"
        limit: int = 20
    ) -> PaginatedResponse[WebhookDelivery]:
        """List webhook delivery attempts."""
        
    def get_delivery(self, delivery_id: UUID) -> WebhookDelivery:
        """Get details of a specific delivery attempt."""

class WebhookDelivery(BaseModel):
    id: UUID
    webhook_id: UUID
    event_type: str
    status: str
    response_code: int | None
    response_body: str | None
    created_at: datetime

Use case

Debugging webhook failures - seeing what was sent and what the response was.

Note

This depends on #10 (Add Webhooks resource) being implemented first.