feat(maintenance): add a maintenance notification during backend migration

src/interfaces/api/routes.rs

@@ -687,13 +687,24 @@ pub fn create_api_routes(app_state: &Arc<AppState>) -> Router<Arc<AppState>> {
     // them on every overlapping request.
     router = router.route("/{*rest}", any(api_not_found));
 
-    // No per-router layers: the global `TraceLayer` + request-id stack in
-    // `main.rs` wraps the whole app (this `/api` router is nested into it),
-    // so a second `TraceLayer` here just double-wrapped every `/api`
-    // request in a redundant span + response-future poll (benches/ROUND13.md
-    // §H1). Compression is likewise the global layer's job — re-applying it
-    // here (no predicate) would compress media downloads, burning CPU for
-    // ~0 gain and stripping `Content-Length`.
+    // Server-status header. Stamps `X-Server-Status` on every
+    // response so the frontend's fetch wrapper can update a
+    // reactive store — banner shows/hides without polling.
+    // Sub-nanosecond on the hot path (single atomic load), a few
+    // µs on the cold path (only during a running migration). See
+    // `middleware::server_status`.
+    let router = router.layer(axum::middleware::from_fn_with_state(
+        app_state.clone(),
+        crate::interfaces::middleware::server_status::server_status_middleware,
+    ));
+
+    // No per-router layers beyond that: the global `TraceLayer` + request-id
+    // stack in `main.rs` wraps the whole app (this `/api` router is nested
+    // into it), so a second `TraceLayer` here just double-wrapped every
+    // `/api` request in a redundant span + response-future poll
+    // (benches/ROUND13.md §H1). Compression is likewise the global layer's
+    // job — re-applying it here (no predicate) would compress media
+    // downloads, burning CPU for ~0 gain and stripping `Content-Length`.
     router
 }