feat: add knowledge retrieval service

backend/apps/knowledge/schemas.py

@@ -65,3 +65,19 @@ class KnowledgeItemOut(Schema):
     source_type: str
     created_at: datetime
     updated_at: datetime
+
+
+class KnowledgeSearchIn(Schema):
+    query: str
+    knowledge_base_id: int | None = None
+    limit: int = 5
+
+
+class KnowledgeSearchResultOut(Schema):
+    id: int
+    knowledge_base_id: int
+    title: str
+    source_type: str
+    content: str
+    score: float
+    metadata: dict