PERF(observability): add HA for Loki and Tempo

- Loki: replicas 1→2 with soft anti-affinity
- Tempo: replicas 1→2 with soft anti-affinity
- Thanos/Prometheus: keep replica 1

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

prometheus/helm-values.yaml

@@ -70,7 +70,7 @@ prometheus:
     # Enable remote write receiver for OTel Collector
     enableRemoteWriteReceiver: true
 
-    # Single replica (HA removed for resource optimization)
+    # Single replica
     replicas: 1
     replicaExternalLabelName: prometheus_replica