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.toml

@@ -31,6 +31,7 @@ clap = { version = "=4.6.1", features = ["derive"] }
 signal-hook = "=0.3.18"
 anyhow = "=1.0.102"
 thiserror = "=2.0.18"
+http-body-util = "=0.1.3"
 futures = "=0.3.31"
 
 [dev-dependencies]