feat(monitoring): add Grafana dashboards for PostgreSQL, Redis, System, App

- nexus-postgresql.json: connections, transactions, cache hit, WAL,
  slow queries, table bloat
- nexus-redis.json: memory, commands/sec, clients, cache hit,
  keyspace hits/misses, evicted keys
- nexus-system.json: CPU, memory, disk, network, load average
- nexus-gunicorn.json: request rate, response time, workers,
  5xx errors, memory per worker
- dashboards.yml: auto-provisioning config

docker/grafana/provisioning/dashboards/nexus-system.json

@@ -0,0 +1,164 @@
+{
+  "uid": "nexus-system",
+  "title": "Nexus — System",
+  "tags": ["node", "system"],
+  "timezone": "browser",
+  "schemaVersion": 36,
+  "refresh": "30s",
+  "time": {
+    "from": "now-1h",
+    "to": "now"
+  },
+  "templating": {
+    "list": [
+      {
+        "name": "datasource",
+        "type": "datasource",
+        "query": "prometheus",
+        "current": {
+          "selected": false,
+          "text": "Prometheus",
+          "value": "Prometheus"
+        }
+      }
+    ]
+  },
+  "panels": [
+    {
+      "id": 1,
+      "title": "CPU Usage %",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 0, "y": 0},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "100 - (avg by(instance) (rate(node_cpu_seconds_total{mode=\"idle\"}[5m])) * 100)",
+          "legendFormat": "CPU %",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "percent",
+          "min": 0,
+          "max": 100
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 2,
+      "title": "Memory Usage %",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 12, "y": 0},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "100 * (1 - node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes)",
+          "legendFormat": "Memory %",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "percent",
+          "min": 0,
+          "max": 100
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 3,
+      "title": "Disk Usage %",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 0, "y": 8},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "100 * (1 - node_filesystem_avail_bytes{fstype!~\"tmpfs|ramfs\"} / node_filesystem_size_bytes{fstype!~\"tmpfs|ramfs\"})",
+          "legendFormat": "{{mountpoint}}",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "percent",
+          "min": 0,
+          "max": 100
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 4,
+      "title": "Network I/O",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 12, "y": 8},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "rate(node_network_receive_bytes_total[5m])",
+          "legendFormat": "Receive {{device}}",
+          "refId": "A"
+        },
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "rate(node_network_transmit_bytes_total[5m])",
+          "legendFormat": "Transmit {{device}}",
+          "refId": "B"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "Bps",
+          "min": 0
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 5,
+      "title": "Load Average",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 24, "x": 0, "y": 16},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "node_load1",
+          "legendFormat": "1m load",
+          "refId": "A"
+        },
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "node_load5",
+          "legendFormat": "5m load",
+          "refId": "B"
+        },
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "node_load15",
+          "legendFormat": "15m load",
+          "refId": "C"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "short",
+          "min": 0
+        },
+        "overrides": []
+      }
+    }
+  ]
+}