Add Overseerr (snap) and Syncthing (apt) to media_stack role so they're
managed by Ansible instead of being manually installed.
- Overseerr: snap install from latest/beta channel (matches current)
- Syncthing: apt install, enable syncthing@pez systemd user service
- slskd: remove stale 'currently stopped' comment from compose file
- london-b host_vars: document snap and apt user services
Fixes PESO-106