feat(synapse): add deployment method virtualenv

roles/synapse/README.md

@@ -20,10 +20,21 @@ The following variables need to be populated:
 
 - `docker`
 - `podman`
+- `virtualenv` - Python virtual env supervised with `systemd`
 
 Set `synapse_deployment_method` to one of the supported deployment methods.
 The current default is `docker`.
 
-### Planned deployment methods
+### `virtualenv` deployment method
 
-- `venv` - Python virtual env supervised with `systemd`
+This deployment method installs a `systemd` service called `synapse.service` to
+control the homeserver process. The service depends on the `network.target` by
+default (see [`synapse_systemd_unit_after`](synapse/main/systemd.yml)), and
+uses the `default.target` as it's `WantedBy`
+(see [`synapse_systemd_install_wanted_by`](synapse/main/systemd.yml)).
+
+To only start synapse after, for example, services for redis and postgresql are up,
+set `synapse_systemd_unit_wants: [ "postgresql.service", "redis.service" ]`.
+
+> [!NOTE]
+> Requires `systemd >= 245` on the target machine