Fix Synapse usage statistics reporting when synapse-usage-exporter is not used

Since the introduction of the synapse-usage-exporter role (55f869254, #3442), group_vars/matrix_servers was overriding matrix_synapse_report_stats_endpoint with an empty string whenever synapse-usage-exporter was disabled, clobbering the role's matrix.org default.

For deployments that enable matrix_synapse_report_stats, Synapse would then try reporting statistics to an empty URL, failing with:

> synapse.app.homeserver - 210 - WARNING - phone_stats_home-0 - Error reporting stats: Invalid URI b''

The default endpoint URL now lives in a dedicated matrix_synapse_report_stats_endpoint_default variable, which group_vars/matrix_servers falls back to when synapse-usage-exporter is not enabled.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

group_vars/matrix_servers

@@ -4931,7 +4931,7 @@ matrix_synapse_ext_synapse_http_antispam_config_async: "{{ matrix_bot_draupnir_s
 
 # Enable Synapse statistics reporting when using synapse-usage-exporter
 matrix_synapse_report_stats: "{{ matrix_synapse_usage_exporter_enabled }}"
-matrix_synapse_report_stats_endpoint: "{{ (('http://' + matrix_synapse_usage_exporter_identifier + ':' + matrix_synapse_usage_exporter_container_port | string + '/report-usage-stats/push') if matrix_synapse_usage_exporter_enabled else '') }}"
+matrix_synapse_report_stats_endpoint: "{{ (('http://' + matrix_synapse_usage_exporter_identifier + ':' + matrix_synapse_usage_exporter_container_port | string + '/report-usage-stats/push') if matrix_synapse_usage_exporter_enabled else matrix_synapse_report_stats_endpoint_default) }}"
 
 matrix_synapse_matrix_authentication_service_enabled: "{{ matrix_authentication_service_enabled }}"
 matrix_synapse_matrix_authentication_service_endpoint: "{{ matrix_authentication_service_http_base_container_url if matrix_authentication_service_enabled else '' }}"

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

@@ -573,7 +573,10 @@ matrix_synapse_federation_listener_resource_names: "{{ ['federation'] if matrix_
 matrix_synapse_report_stats: false
 
 # The endpoint to report homeserver usage statistics to.
-matrix_synapse_report_stats_endpoint: "https://matrix.org/report-usage-stats/push"
+matrix_synapse_report_stats_endpoint: "{{ matrix_synapse_report_stats_endpoint_default }}"
+
+# The default endpoint to report homeserver usage statistics to (the one operated by matrix.org).
+matrix_synapse_report_stats_endpoint_default: "https://matrix.org/report-usage-stats/push"
 
 # Controls whether the Matrix server will track presence status (online, offline, unavailable) for users.
 # If users participate in large rooms with many other servers,