Align backend plan with codebase

lib/server/rateLimit.ts

@@ -0,0 +1,20 @@
+/**
+ * In-memory rate limiter (per server instance). Sufficient for small deploys;
+ * replace with Redis or edge limits if you scale horizontally.
+ */
+
+const windows = new Map<string, number>();
+
+export function rateLimitKey(
+  key: string,
+  minIntervalMs: number,
+): { ok: true } | { ok: false; retryAfterMs: number } {
+  const now = Date.now();
+  const last = windows.get(key) ?? 0;
+  const elapsed = now - last;
+  if (elapsed < minIntervalMs) {
+    return { ok: false, retryAfterMs: minIntervalMs - elapsed };
+  }
+  windows.set(key, now);
+  return { ok: true };
+}