feat(dashboard): a curated default layout, merged duplicate widgets, a labelled Overview (#5213)

- New installs start from widgets.RECOMMENDED, packed row by row for the
  enabled plugins; the edit drawer can reset a dashboard to it.
- Host Agent — Resources folds into Hosts — Overview (which gains load 1m),
  UniFi — Devices into UniFi — Overview; migration 0032 moves existing
  dashboards onto the survivors.
- The Overview labels what it counts (hosts, monitors up/down/pending,
  alerts firing) and lists firing alerts and down monitors with since-times.
- Uptime polls every 5 minutes (poll_seconds) instead of every cycle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/hosts/service.py

@@ -16,7 +16,7 @@ from steward.models.monitors import Monitor, MonitorResult
 
 # Host-level agent metrics read for the at-a-glance views (plus the root
 # mount's disk_used_pct, read from the "<host>:/" sub-resource).
-AGENT_GLANCE_METRICS = ("cpu_pct", "mem_used_pct", "disk_used_pct", "disk_used_pct_worst")
+AGENT_GLANCE_METRICS = ("cpu_pct", "mem_used_pct", "disk_used_pct", "disk_used_pct_worst", "load_1m")
 
 
 async def check_name_free(db, name: str, exclude_id: str | None = None) -> None:
@@ -172,7 +172,7 @@ async def agent_overview_by_host(
 ) -> dict[str, dict]:
     """Per-host agent glance (cpu/mem/disk + freshness) from plugin_metric_latest.
 
-    {host_name: {cpu_pct, mem_used_pct, disk_used_pct_worst, disk_root,
+    {host_name: {cpu_pct, mem_used_pct, disk_used_pct_worst, disk_root, load_1m,
                  last_at, fresh}} for each host with any agent samples.
 
     Core-safe (no host_agent import). Freshness compares the latest host-level