Fix SSE route dependency and align architecture docs

backend/app/domain/retrieval/__init__.py

@@ -0,0 +1,8 @@
+"""Initialize the app.domain.retrieval package."""
+
+from .models import RetrievalQuery, RetrievedChunk
+from .ports import EmbeddingProvider, Retriever, VectorIndex
+# Keep package boundaries explicit so backend imports stay predictable.
+
+
+__all__ = ["RetrievalQuery", "RetrievedChunk", "EmbeddingProvider", "Retriever", "VectorIndex"]