Add brain memory services and APIs

Introduce the backend pieces for brain memory ingestion, routing, and
system telemetry so the new knowledge workflows can project data into a
brain view. The supporting tests lock in the new behavior and keep the
expanded backend surface stable.

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

backend/app/services/llm_service.py

@@ -5,11 +5,14 @@ OpenAI / Claude / Ollama / DeepSeek / 任意 OpenAI 兼容接口
 
 from abc import ABC, abstractmethod
 from typing import AsyncIterator
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
 from langchain_core.messages import BaseMessage, AIMessage
 from langchain_openai import ChatOpenAI
 from langchain_anthropic import ChatAnthropic
 from langchain_ollama import ChatOllama
 from app.config import settings
+from app.models.user import User
 import httpx
 import os