Add restart: unless-stopped to backend and frontend

A frontend OOM during heavy LTspice image builds left it crashed with no
auto-recovery (Caddy served empty 200s). Restart policy self-heals crashes
and survives host reboots.

docker-compose.yml

@@ -2,6 +2,7 @@ services:
   backend:
     build:
       context: ./backend
+    restart: unless-stopped
     env_file: .env
     volumes:
       - ./notebooks:/app/notebooks
@@ -14,6 +15,7 @@ services:
   frontend:
     build:
       context: ./frontend
+    restart: unless-stopped
     env_file: .env
     environment:
       - PUBLIC_API_URL=https://${SPICEBOOK_DOMAIN:-localhost:4321}