Reverse proxy / upstream balancer (Middlewares — P2)

[meshin-dev]

• What: HTTP/WS reverse proxy with policy hooks (modify request/response, health checks minimal, simple balancing strategies).
• Why Middlewares: Integration/ops feature; heavier config.
• Precedents: Echo Proxy; Fiber Proxy; Gin examples and third‑party middleware. (https://echo.labstack.com/docs/middleware/proxy, https://docs.gofiber.io/api/middleware/proxy/, https://github.com/chenhg5/gin-reverseproxy)