feat: grouped server cards with progress bars and sparklines

- Agent sends group name (defaults to hostname) and richer metric notes
  (memory/disk include absolute GB values; CPU note includes system uptime)
- push() accepts group param; server stores it as agent_group on deployments
- Upsert key is now (company_id, agent_group, name) so two agents on the
  same company but different servers don't overwrite each other's CPU/Memory
- Add widget config packs: linux-base, web-nginx, database-postgres/redis,
  docker-host, custom-cmd examples

widgets/docker-host.yml

@@ -0,0 +1,20 @@
+# Docker host — disk for docker data dir + Docker daemon check.
+
+widgets:
+  - type: system_disk
+    name: Docker disk
+    path: /var/lib/docker
+    degrade_threshold: 75
+    offline_threshold: 90
+
+  - type: custom_cmd
+    name: Docker daemon
+    cmd: docker info > /dev/null 2>&1 && exit 0 || exit 2
+
+  # Optional: count running containers (degraded if 0, offline if docker is down).
+  # - type: custom_cmd
+  #   name: Containers running
+  #   cmd: |
+  #     COUNT=$(docker ps -q 2>/dev/null | wc -l)
+  #     [ $? -ne 0 ] && exit 2
+  #     [ "$COUNT" -eq 0 ] && exit 1 || exit 0