feat(http): implement shared HTTP client (ClientWithMiddleware + retry + Retry-After, OQ-40)

Adds SharedHttpClient wrapping ArcSwap<ClientWithMiddleware> with a
RetryTransientMiddleware + inlined RetryAfterMiddleware stack.
HttpClientConfig covers pool, timeout, retry policy, and optional CA
bundle/client cert. reload() rebuilds and swaps via ArcSwap. No env-var
reads; credential injection is per-request, not at construction.

crates/alknet-http/src/client/mod.rs

@@ -3,4 +3,8 @@
 //!
 //! See `docs/architecture/crates/http/http-adapters.md` and OQ-40.
 
-// TODO: implement
+mod http_client;
+mod retry_after;
+
+pub use http_client::{ClientCertConfig, HttpClientBuildError, HttpClientConfig, SharedHttpClient};
+pub use retry_after::RetryAfterMiddleware;