Fix SSE route dependency and align architecture docs

backend/app/application/knowledge/services.py

@@ -0,0 +1,19 @@
+"""Implement application-layer logic for services."""
+
+from __future__ import annotations
+
+from app.domain.retrieval import RetrievalQuery, Retriever, RetrievedChunk
+# Keep orchestration logic centralized so use-case flow stays easy to trace.
+
+
+
+class KnowledgeRetrievalService:
+    """Provide the Knowledge Retrieval Service service."""
+    def __init__(self, *, retriever: Retriever) -> None:
+        """Initialize the Knowledge Retrieval Service instance."""
+        self.retriever = retriever
+
+    def retrieve(self, *, query: str, top_k: int, filters: str | None = None) -> list[RetrievedChunk]:
+        """Handle retrieve for the Knowledge Retrieval Service instance."""
+        retrieval_query = RetrievalQuery(query=query, top_k=top_k, filters=filters)
+        return self.retriever.retrieve(retrieval_query)