refactor: break up monolithic routes.py into modular domain structure

application/api/user/sources/__init__.py

@@ -0,0 +1,7 @@
+"""Sources module."""
+
+from .chunks import sources_chunks_ns
+from .routes import sources_ns
+from .upload import sources_upload_ns
+
+__all__ = ["sources_ns", "sources_chunks_ns", "sources_upload_ns"]