PERF(observability): reduce replicas, add priority

- Reduce Prometheus replicas from 2 to 1
- Reduce Grafana replicas from 2 to 1
- Reduce Blackbox-exporter replicas from 2 to 1
- Move Loki, Thanos, Tempo to workers (remove tolerations)
- Add medium-priority to Prometheus, Loki, Thanos, Tempo

prometheus/helm-values.yaml

@@ -70,10 +70,13 @@ prometheus:
     # Enable remote write receiver for OTel Collector
     enableRemoteWriteReceiver: true
 
-    # HA: 2 replicas on different worker nodes
-    replicas: 2
+    # Single replica (HA removed for resource optimization)
+    replicas: 1
     replicaExternalLabelName: prometheus_replica
 
+    # Medium priority for observability
+    priorityClassName: medium-priority
+
     # Pod anti-affinity for HA
     affinity:
       podAntiAffinity: