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/routers/folder.py

@@ -6,7 +6,7 @@ from app.database import get_db
 from app.models.folder import Folder
 from app.models.user import User
 from app.schemas.folder import FolderCreate, FolderUpdate, FolderOut, FolderTreeOut
-from app.services.auth_service import get_current_user
+from app.routers.auth import get_current_user
 
 router = APIRouter(prefix="/api/folders", tags=["文件夹"])