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/system.py

@@ -0,0 +1,9 @@
+from fastapi import APIRouter
+from app.services.system_service import SystemService
+
+router = APIRouter(prefix='/api/system', tags=['system'])
+
+
+@router.get('/status')
+async def get_system_status():
+    return SystemService().get_status()