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-gunicorn.json

@@ -0,0 +1,149 @@
+{
+  "uid": "nexus-app",
+  "title": "Nexus — Application",
+  "tags": ["gunicorn", "flask"],
+  "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": "Request Rate (nginx)",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 0, "y": 0},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "rate(nginx_http_requests_total[5m])",
+          "legendFormat": "Requests/sec",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "reqps",
+          "min": 0
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 2,
+      "title": "Response Time (nginx)",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 12, "y": 0},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "histogram_quantile(0.95, sum(rate(nginx_http_request_duration_seconds_bucket[5m])) by (le))",
+          "legendFormat": "p95",
+          "refId": "A"
+        },
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "histogram_quantile(0.99, sum(rate(nginx_http_request_duration_seconds_bucket[5m])) by (le))",
+          "legendFormat": "p99",
+          "refId": "B"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "s",
+          "min": 0
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 3,
+      "title": "Active Workers",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 0, "y": 8},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "count by (instance) (node_processes_state{state=\"S\", cmdline=~\".*gunicorn.*\"})",
+          "legendFormat": "Workers {{instance}}",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "short",
+          "min": 0
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 4,
+      "title": "5xx Errors",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 12, "x": 12, "y": 8},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "rate(nginx_http_requests_total{status=~\"5..\"}[5m])",
+          "legendFormat": "5xx/sec",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "reqps",
+          "min": 0
+        },
+        "overrides": []
+      }
+    },
+    {
+      "id": 5,
+      "title": "Memory per Worker",
+      "type": "timeseries",
+      "gridPos": {"h": 8, "w": 24, "x": 0, "y": 16},
+      "datasource": {"type": "prometheus", "uid": "${datasource}"},
+      "targets": [
+        {
+          "datasource": {"type": "prometheus", "uid": "${datasource}"},
+          "expr": "process_resident_memory_bytes{cmdline=~\".*gunicorn.*\"} / 1024 / 1024",
+          "legendFormat": "{{cmdline}}",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "custom": {"drawStyle": "line", "lineWidth": 2, "fillOpacity": 10},
+          "unit": "mbytes",
+          "min": 0
+        },
+        "overrides": []
+      }
+    }
+  ]
+}