Implement body size limit middleware with dynamic config reload

Add body_limit middleware that reads limit from ArcSwap<DynamicConfig>
on each request, enabling runtime config changes without restart.
Uses Content-Length header check for fast rejection and http_body_util::Limited
for streaming body enforcement. Default limit: 100 MB (104,857,600 bytes).
Returns 413 Payload Too Large when exceeded.

Cargo.lock

@@ -1585,6 +1585,7 @@ dependencies = [
  "axum",
  "clap",
  "futures",
+ "http-body-util",
  "hyper",
  "rcgen",
  "reqwest",