Add server MaxBodySize middleware to prevent memory exhaustion

Wraps request body with http.MaxBytesReader to limit incoming payload
size. Without this, any endpoint accepting a body is vulnerable to
large uploads consuming all available memory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/middleware_bodylimit.go

@@ -0,0 +1,15 @@
+package server
+
+import "net/http"
+
+// MaxBodySize returns a middleware that limits the size of incoming request
+// bodies. If the body exceeds n bytes, the server returns 413 Request Entity
+// Too Large. It wraps the body with http.MaxBytesReader.
+func MaxBodySize(n int64) Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			r.Body = http.MaxBytesReader(w, r.Body, n)
+			next.ServeHTTP(w, r)
+		})
+	}
+}