fix(monitoring): correct postgres configuration change query

changes() produced one series per pg_settings_* metric with identical labels,
so VictoriaMetrics rejected the rule with "duplicate output timeseries" whenever
a setting changed.

Assisted-by: DeepSeek Harness (deepseek-flash)

hosts/12kingdoms-youko/homelab-services/monitoring/alert_rules/postgres-exporter.yml

@@ -176,8 +176,8 @@ groups:
 
       - alert: PostgresqlConfigurationChanged
         expr:
-          'changes({__name__=~"pg_settings_.*", __name__!="pg_settings_transaction_read_only"}[5m])
-          > 0'
+          'sum(changes({__name__=~"pg_settings_.*",
+          __name__!="pg_settings_transaction_read_only"}[5m])) > 0'
         for: 0m
         labels:
           severity: info