Keep 200 status for SPA fallback

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/crates/api/src/lib.rs

@@ -63,7 +63,7 @@ impl AppState {
 
 pub fn build_app(state: AppState) -> Router {
     let index = format!("{}/index.html", state.cfg.frontend_dir);
-    let spa = ServeDir::new(&state.cfg.frontend_dir).not_found_service(ServeFile::new(index));
+    let spa = ServeDir::new(&state.cfg.frontend_dir).fallback(ServeFile::new(index));
     Router::new()
         .route("/healthz", get(healthz))
         .route("/api/openapi.json", get(openapi::spec))