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/linux-base.yml

@@ -0,0 +1,19 @@
+# Linux base — CPU, memory and disk.
+# Paste into your ethica-agent.yml widgets section.
+
+widgets:
+  - type: system_cpu
+    name: CPU
+    degrade_threshold: 80
+    offline_threshold: 95
+
+  - type: system_memory
+    name: Memory
+    degrade_threshold: 85
+    offline_threshold: 97
+
+  - type: system_disk
+    name: Disk /
+    path: /
+    degrade_threshold: 80
+    offline_threshold: 95