fix(cli): route tracing diagnostics to stderr

Machine output (the import/seed-dev subcommands' JSON result) now owns stdout;
tracing logs go to stderr. Lets a caller parse the import outcome without
log-line contamination on stdout. No behavioral change to the server beyond
where its logs are written.

src/main.rs

@@ -12,7 +12,9 @@ async fn main() -> Result<()> {
             EnvFilter::try_from_default_env()
                 .unwrap_or_else(|_| "openfut_core=debug,tower_http=debug".into()),
         )
-        .with(tracing_subscriber::fmt::layer())
+        // Diagnostics on stderr so stdout carries only machine output (the
+        // `import`/`seed-dev` subcommands print a clean JSON result there).
+        .with(tracing_subscriber::fmt::layer().with_writer(std::io::stderr))
         .init();
 
     let cfg = config::Config::from_env()?;