feat(server): BLE 网关独立限流 — 每网关 60 req/60s

为 /health/gateway 路由添加 gateway_id 级别的速率限制，
网关认证（API Key）→ 限流检查 → handler 三层中间件。
Redis 不可达时同样遵循 fail_close 策略。

crates/erp-server/src/main.rs

@@ -750,6 +750,10 @@ async fn main() -> anyhow::Result<()> {
         .nest(
             "/health/gateway",
             erp_health::HealthModule::gateway_routes()
+                .layer(axum::middleware::from_fn_with_state(
+                    state.clone(),
+                    middleware::rate_limit::rate_limit_by_gateway,
+                ))
                 .layer(axum::middleware::from_fn_with_state(
                     state.clone(),
                     erp_health::gateway_auth::gateway_auth_middleware,