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/dashboards.yml

@@ -0,0 +1,11 @@
+apiVersion: 1
+providers:
+  - name: 'nexus-dashboards'
+    orgId: 1
+    folder: 'Nexus'
+    type: file
+    disableDeletion: false
+    updateIntervalSeconds: 10
+    allowUiUpdates: true
+    options:
+      path: /etc/grafana/provisioning/dashboards