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/web-nginx.yml

@@ -0,0 +1,22 @@
+# Nginx web server — process check + HTTP/HTTPS endpoint + ports.
+# Replace MY_DOMAIN with the actual domain.
+
+widgets:
+  - type: process_check
+    name: nginx
+    process: nginx
+
+  - type: http_check
+    name: HTTPS
+    url: https://MY_DOMAIN
+    timeout: 10
+
+  - type: port_check
+    name: Port 80
+    host: localhost
+    port: 80
+
+  - type: port_check
+    name: Port 443
+    host: localhost
+    port: 443