refactor: reorganize project into monorepo with frontend scaffolding

Convert project from single backend to monorepo structure with separate
frontend (Vue 3 + TypeScript + Vite) and backend directories. Updates
all configuration files and build system to support both workspaces.

Ref: T007 (specs/001-modbus-relay-control)

backend/src/middleware/mod.rs

@@ -0,0 +1,5 @@
+//! Custom middleware for the application.
+//!
+//! This module contains custom middleware implementations including rate limiting.
+
+pub mod rate_limit;