feat: add metrics, observability, test fixtures, and scaling playbook artifacts

monitoring/grafana/dashboard.json

@@ -34,6 +34,33 @@
       "title": "Chat Scrape Health",
       "type": "stat",
       "targets": [{ "expr": "up{job=\"chat\"}" }]
+    },
+    {
+      "title": "Cache Hit Ratio",
+      "type": "timeseries",
+      "targets": [
+        {
+          "expr": "linearvoid_cache_hit_ratio"
+        }
+      ]
+    },
+    {
+      "title": "Redis Memory Usage",
+      "type": "timeseries",
+      "targets": [
+        {
+          "expr": "linearvoid_chat_redis_memory_usage_ratio"
+        }
+      ]
+    },
+    {
+      "title": "Chat Request Latency p95",
+      "type": "timeseries",
+      "targets": [
+        {
+          "expr": "histogram_quantile(0.95, sum(rate(linearvoid_chat_request_duration_seconds_bucket[5m])) by (le))"
+        }
+      ]
     }
   ]
 }