Use docker inspect for Synapse systemd health check and lower health interval

Switch the systemd ExecStartPost health check from docker exec + curl
to polling docker inspect for container health status. This piggybacks
on the container image's built-in HEALTHCHECK instead of duplicating it.

Also add a configurable container health interval (5s for Traefik setups,
15s otherwise) to speed up startup readiness detection without affecting
non-Traefik deployments.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

roles/custom/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2

@@ -33,6 +33,7 @@ ExecStartPre={{ devture_systemd_docker_base_host_command_docker }} create \
 			--read-only \
 			--tmpfs=/tmp:rw,noexec,nosuid,size={{ matrix_synapse_tmp_directory_size_mb }}m \
 			--network={{ matrix_synapse_container_network }} \
+			--health-interval={{ matrix_synapse_container_health_interval }} \
 			{% if matrix_synapse_container_client_api_host_bind_port %}
 			-p {{ matrix_synapse_container_client_api_host_bind_port }}:{{ matrix_synapse_container_client_api_port }} \
 			{% endif %}