Fix streaming TransferEncodingError

DB saves were happening inside the generator after [DONE], causing
connection abort if any await failed. Moved to BackgroundTask.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.py

@@ -129,22 +129,28 @@ async def chat_completions(req: ChatRequest, bg: BackgroundTasks):
                          if m.role == "user"), "")
 
     if req.stream:
+        collected: list[str] = []
+
         async def gen() -> AsyncIterator[bytes]:
-            full_response = []
-            async for chunk in oll.chat_stream(messages, model=req.model):
-                full_response.append(chunk)
-                data = json.dumps({"choices": [{"delta": {"content": chunk},
-                                               "finish_reason": None}]})
-                yield f"data: {data}\n\n".encode()
-            yield b"data: [DONE]\n\n"
-            # save after stream
-            response_text = "".join(full_response)
-            await mem.save_turn(req.session_id, "user",    user_content)
+            try:
+                async for chunk in oll.chat_stream(messages, model=req.model):
+                    collected.append(chunk)
+                    data = json.dumps({"choices": [{"delta": {"content": chunk},
+                                                   "finish_reason": None}]})
+                    yield f"data: {data}\n\n".encode()
+                yield b"data: [DONE]\n\n"
+            except Exception as e:
+                yield f"data: {json.dumps({'error': str(e)})}\n\n".encode()
+
+        async def _save_stream():
+            response_text = "".join(collected)
+            await mem.save_turn(req.session_id, "user",      user_content)
             await mem.save_turn(req.session_id, "assistant", response_text)
-            await mem.extract_and_store(user_content,    user_id=req.user_id)
-            await mem.extract_and_store(response_text,   user_id=req.user_id)
+            await mem.extract_and_store(user_content,  user_id=req.user_id)
+            await mem.extract_and_store(response_text, user_id=req.user_id)
             await mem.summarize_old_turns(req.session_id)
 
+        bg.add_task(_save_stream)
         return StreamingResponse(gen(), media_type="text/event-stream")
 
     response_text = await oll.chat(messages, model=req.model)