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/database-redis.yml

@@ -0,0 +1,17 @@
+# Redis — process check + port + optional ping.
+
+widgets:
+  - type: process_check
+    name: Redis
+    process: redis-server
+
+  - type: port_check
+    name: Redis Port
+    host: localhost
+    port: 6379
+    timeout: 3
+
+  # Optional: PING check (requires redis-cli).
+  # - type: custom_cmd
+  #   name: Redis PING
+  #   cmd: redis-cli ping | grep -q PONG && exit 0 || exit 2