Add sonarr service directory with README

Sonarr is running on london-b as an apt-managed systemd service
but was the only *arr service without a services/ directory in the
repo. Add services/sonarr/README.md documenting the install method,
data paths, and how it differs from the other *arr services.

Closes PESO-133

ansible/services/sonarr/README.md

@@ -0,0 +1,18 @@
+# Sonarr
+
+TV series management and automated downloading.
+
+## Deployment
+
+- **Host:** london-b
+- **Install method:** APT package (`sonarr` v3, mono-based)
+- **Service:** `sonarr.service` (package-managed unit file — do not override)
+- **Data directory:** `/var/lib/sonarr`
+- **Web UI:** `sonarr.pez.sh` (proxied via Caddy on helsinki-a)
+- **Managed by:** `media_stack` Ansible role (ensures service is enabled and started)
+
+## Notes
+
+Unlike radarr/lidarr/readarr/prowlarr (which use manually installed binaries with custom unit files), sonarr is installed via APT and its systemd unit is owned by the package. Use `dpkg-reconfigure -plow sonarr` to change User/Group/UMask settings rather than editing the unit file directly.
+
+The `media_stack` role also sets up a midnight cron restart (`systemctl restart sonarr`).