feat(saas): add quota check middleware for relay requests

Injects billing quota verification before relay chat completion requests.
Checks monthly relay_requests quota via billing::service::check_quota.
Gracefully degrades on quota service failure (logs warning, allows request).

crates/zclaw-saas/src/main.rs

@@ -368,6 +368,10 @@ async fn build_router(state: AppState) -> axum::Router {
             state.clone(),
             zclaw_saas::middleware::request_id_middleware,
         ))
+        .layer(middleware::from_fn_with_state(
+            state.clone(),
+            zclaw_saas::middleware::quota_check_middleware,
+        ))
         .layer(middleware::from_fn_with_state(
             state.clone(),
             zclaw_saas::middleware::rate_limit_middleware,