Fix: retrieve once per document chat question (#14673)

Document chat searched the vector store twice per question: once
explicitly to build the reference list, and again inside
RetrieverQueryEngine.query(), which retrieves before synthesizing. That
meant two query-embedding calls and two full vector searches each time.

The engine added nothing beyond the retriever call (no node
postprocessors are configured), so drop it and synthesize directly from
the nodes already retrieved. The streaming response is unchanged.

src/paperless_ai/chat.py

@@ -124,7 +124,6 @@ def _stream_chat_with_documents(
         return
 
     from llama_index.core.prompts import PromptTemplate
-    from llama_index.core.query_engine import RetrieverQueryEngine
     from llama_index.core.response_synthesizers import get_response_synthesizer
     from llama_index.core.retrievers import VectorIndexRetriever
 
@@ -142,9 +141,8 @@ def _stream_chat_with_documents(
             str(pk) for pk in documents.values_list("pk", flat=True)
         )
 
-    # Hold the shared read lock for the whole operation: the query engine
-    # retrieves from the vector store again during synthesis, so the connection
-    # must stay open (and the swap must not run) until the stream finishes.
+    # Hold the shared read lock for the whole operation so the compaction swap
+    # cannot replace the database file while this connection is open.
     with read_store() as store:
         index = load_or_build_index(config, store)
         retriever = VectorIndexRetriever(
@@ -179,19 +177,13 @@ def _stream_chat_with_documents(
             refine_template=refine_template,
             streaming=True,
         )
-        query_engine = RetrieverQueryEngine.from_args(
-            retriever=retriever,
-            llm=client.llm,
-            response_synthesizer=response_synthesizer,
-            streaming=True,
-        )
 
         logger.debug("Document chat query: %s", query_str)
         # Release the pooled DB connection for the slow streaming LLM response
         # so it is not pinned for the whole stream; see paperless_ai.db and
         # #12976.
         with db_connection_released():
-            response_stream = query_engine.query(query_str)
+            response_stream = response_synthesizer.synthesize(query_str, top_nodes)
             for chunk in response_stream.response_gen:
                 yield chunk
                 sys.stdout.flush()