What is a `SecurityFilterChain`, and where does it sit in a request's lifecycle?

It's an ordered list of servlet filters (CORS handling, CSRF checks, authentication, authorization) that every incoming HTTP request passes through before it ever reaches your `@RestController`. You define it as a `@Bean` returning `HttpSecurity`-built configuration, and Spring Boot can support multiple chains matched by URL pattern (e.g. a stateless chain for `/api/**`, a stateful one for `/admin/**`).