feat(mw): add dev-mode rate limiting bypass

Add build tag split: real rate limiter under !dev, no-op under dev build tag.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/mw/ratelimit_dev.go

@@ -0,0 +1,17 @@
+//go:build dev
+// +build dev
+
+package mw
+
+import (
+	"net/http"
+	"time"
+)
+
+func RateLimit(limit int, window time.Duration) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			next.ServeHTTP(w, r)
+		})
+	}
+}