feat: prepare RAG evidence and compatible chat requests

backend/app/models/__init__.py

@@ -2,6 +2,7 @@ from app.models.auth import AuthRateLimit, AuthSession, User
 from app.models.catalog import AuditEvent, Document, Source
 from app.models.indexing import Chunk, IndexPublication, IndexState, IndexVersion
 from app.models.knowledge import KnowledgeItem
+from app.models.rag import RagContext
 from app.models.retrieval import RetrievalRun
 from app.models.vectors import (
     ChunkVector,
@@ -23,6 +24,7 @@ __all__ = [
     "IndexState",
     "IndexVersion",
     "KnowledgeItem",
+    "RagContext",
     "RetrievalRun",
     "Source",
     "User",