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.

src/proxy/mod.rs

@@ -1,3 +1,20 @@
+pub mod body_limit;
 pub mod error;
 pub mod handler;
 pub mod headers;
+
+use std::sync::Arc;
+
+use arc_swap::ArcSwap;
+
+use crate::config::DynamicConfig;
+
+pub fn router_with_body_limit(
+    router: axum::Router,
+    config: Arc<ArcSwap<DynamicConfig>>,
+) -> axum::Router {
+    router.layer(axum::middleware::from_fn_with_state(
+        config,
+        body_limit::body_limit_middleware,
+    ))
+}