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)
+		})
+	}
+}

server/middleware_bodylimit_test.go

@@ -0,0 +1,61 @@
+package server_test
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx/server"
+)
+
+func TestMaxBodySize(t *testing.T) {
+	t.Run("allows body within limit", func(t *testing.T) {
+		handler := server.MaxBodySize(1024)(
+			http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				b, err := io.ReadAll(r.Body)
+				if err != nil {
+					http.Error(w, err.Error(), http.StatusBadRequest)
+					return
+				}
+				w.WriteHeader(http.StatusOK)
+				w.Write(b)
+			}),
+		)
+
+		body := strings.NewReader("hello")
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodPost, "/", body)
+		handler.ServeHTTP(w, req)
+
+		if w.Code != http.StatusOK {
+			t.Fatalf("got status %d, want %d", w.Code, http.StatusOK)
+		}
+		if w.Body.String() != "hello" {
+			t.Fatalf("got body %q, want %q", w.Body.String(), "hello")
+		}
+	})
+
+	t.Run("rejects body exceeding limit", func(t *testing.T) {
+		handler := server.MaxBodySize(5)(
+			http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				_, err := io.ReadAll(r.Body)
+				if err != nil {
+					http.Error(w, "body too large", http.StatusRequestEntityTooLarge)
+					return
+				}
+				w.WriteHeader(http.StatusOK)
+			}),
+		)
+
+		body := strings.NewReader("this is longer than 5 bytes")
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodPost, "/", body)
+		handler.ServeHTTP(w, req)
+
+		if w.Code != http.StatusRequestEntityTooLarge {
+			t.Fatalf("got status %d, want %d", w.Code, http.StatusRequestEntityTooLarge)
+		}
+	})
+}