feat(server): re-export URL helpers; callable job done_message

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_mf_url_exports.py

@@ -0,0 +1,8 @@
+import server.mf as smf
+
+
+def test_url_helpers_reexported():
+    assert callable(smf.is_url)
+    assert callable(smf.is_playlist_url)
+    assert callable(smf.download_playlist)
+    assert callable(smf.download_single)