fix(monitoring): fix inhibit_rules snake_case, suppress InfoInhibitor spam

The Prometheus operator uses snake_case (inhibit_rules) not camelCase
(inhibitRules), causing alertmanager reconciliation to fail. Also route
InfoInhibitor alerts to null to stop flooding the Matrix alerts room.

base/monitoring/prometheus-values.yaml

@@ -141,6 +141,9 @@ alertmanager:
             - alertname = Watchdog
           receiver: matrix
           repeat_interval: 12h
+        - matchers:
+            - alertname = InfoInhibitor
+          receiver: "null"
         - matchers:
             - severity = critical
           receiver: matrix
@@ -148,11 +151,12 @@ alertmanager:
             - severity = warning
           receiver: matrix
     receivers:
+      - name: "null"
       - name: matrix
         webhook_configs:
           - url: "http://matrix-alertmanager-receiver.monitoring.svc.cluster.local:3000/alerts/alerts"
             send_resolved: true
-    inhibitRules:
+    inhibit_rules:
       # Critical alerts suppress warnings for the same alertname+namespace
       - source_matchers:
           - severity = critical