Align homeserver/coturn service priorities to avoid first-start cert race

The startup issue came from a timing dependency around coturn TLS certs:

- `matrix-coturn.service` depends on
  `matrix-traefik-certs-dumper-wait-for-domain@<matrix-fqdn>.service`
- That waiter succeeds only after Traefik has obtained and dumped a cert for
  the Matrix hostname (typically driven by homeserver labels/routes becoming
  active)
- If coturn is started too early, it can block/fail waiting for cert files
  that are not yet present

Historically, coturn priority was mode-dependent:

- `one-by-one`: coturn at 1500 (delayed after homeserver)
- other modes: coturn at 900 (before homeserver)

This could still trigger undesirable startup ordering and confusing behavior
in non-`one-by-one` modes, especially during initial bootstrap/restart flows
where cert availability lags service startup.

This change makes ordering explicit and consistent:

1. Introduce `matrix_homeserver_systemd_service_manager_priority` (default 1000)
   in `roles/custom/matrix-base/defaults/main.yml`.
2. Use that variable for the homeserver service entry in
   `group_vars/matrix_servers`.
3. Set coturn priority relative to homeserver priority in all modes:
   `matrix_homeserver_systemd_service_manager_priority + 500`.
4. Update inline documentation comments in `group_vars/matrix_servers` to
   match the new behavior and rationale.

Result:

- Homeserver/coturn ordering is deterministic and mode-agnostic.
- Coturn is intentionally started later than the homeserver by default,
  reducing first-start certificate wait/fail races.
- Priority intent is now centralized and configurable via a dedicated
  homeserver priority variable.
- Coturn may still be stated earlier, because the homeserver typically
  has a `Wants` "dependency" on it, but that's alright

roles/custom/matrix-base/defaults/main.yml

@@ -92,6 +92,10 @@ matrix_homeserver_enabled: true
 # Note that the homeserver implementation of a server will not be able to be changed without data loss.
 matrix_homeserver_implementation: synapse
 
+# The priority that the homeserver starts with (lower = starts earlier).
+# Related to the systemd_service_manager role and `devture_systemd_service_manager_services_list*` variables.
+matrix_homeserver_systemd_service_manager_priority: 1000
+
 # This contains a secret, which is used for generating various other secrets later on.
 matrix_homeserver_generic_secret_key: ''