chore(deploy): add restart: unless-stopped to app container

The app service in docker-compose.yml had no restart policy (defaults to
'no'), so if the process exited -- e.g. the watcher-leak panic fixed in
the previous commit -- the container stayed down until a manual
restart. Adding restart: unless-stopped makes the container self-recover
from crashes or host reboots, while still honoring explicit 'docker
compose down'.

Defense-in-depth alongside the scanner leak/panic fix: even if a future
unforeseen panic occurs, the app comes back automatically.

docker-compose.yml

@@ -31,6 +31,7 @@ services:
   app:
     image: git.linuxhg.com/bookhoard/bookhoard:${IMAGE_TAG:-latest}
     container_name: bookhoard
+    restart: unless-stopped
     environment:
       # Database Configuration
       DATABASE_HOST: db