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/__init__.py

@@ -10,3 +10,5 @@ from app.routers.settings import router as settings_router
 from app.routers.folder import router as folder_router
 from app.routers.skill import router as skill_router
 from app.routers.log import router as log_router
+from app.routers.system import router as system_router
+from app.routers.brain import router as brain_router