Fix SSE route dependency and align architecture docs

backend/app/application/knowledge/__init__.py

@@ -0,0 +1,7 @@
+"""Initialize the app.application.knowledge package."""
+
+from .services import KnowledgeRetrievalService
+# Keep package boundaries explicit so backend imports stay predictable.
+
+
+__all__ = ["KnowledgeRetrievalService"]