Migrate server handlers to Actix-web
Replace custom Context/Request/Response with actix-web AppState and
HttpResponse. Update handlers to accept web::Data, web::Path and
web::Query and return actix_web::Result<HttpResponse>. Add actix-web,
actix-rt and actix-cors deps, use web::block for blocking work, and
adjust cache/update_cache to use Arc<Mutex<...>>. Also add small tracing
imports and Cargo.lock updates.