feat(dashboard): phase C richer panels — host time-series graph + cpu sparklines

Milestone 72 phase C — bring the host-view graphs onto the dashboard:
- host_resource_history widget reworked into a real host-view chart: epoch-ms
  linear axis (no Chart.js date adapter), themed like the host-detail charts,
  maintainAspectRatio:false so it fills the resized panel, unique canvas per
  widget instance (wid), and empty states ("pick a host" / "no metrics yet").
  Was previously unusable — it had a broken time axis and no way to choose a host.
- Add a "host" param type: the edit form renders a live dropdown of hosts
  (dashboard routes now pass the host list to the editor); the chosen host_id is
  stored in config and fed to the widget.
- Hosts-overview widget gains a per-row CPU sparkline (last hour) via the shared
  sparkline_svg helper — the host-view at-a-glance trend, on the main widget.

Charts/sparklines render only in the browser, so CI can't exercise them — needs
an operator visual check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/hosts/routes.py

@@ -153,6 +153,36 @@ async def _agent_overview_by_host(db, host_names: list[str]) -> dict[str, dict]:
     return out
 
 
+async def _agent_cpu_sparks_by_host(db, host_names: list[str], hours: int = 1) -> dict[str, str]:
+    """{host_name: inline-SVG cpu sparkline} over the last `hours` (core-safe).
+
+    A tiny at-a-glance trend in each row — the host-view sparkline, on the widget.
+    """
+    from steward.models.metrics import PluginMetric
+    from steward.core.status import sparkline_svg
+    if not host_names:
+        return {}
+    cutoff = datetime.now(timezone.utc) - timedelta(hours=hours)
+    rows = (await db.execute(
+        select(PluginMetric.resource_name, PluginMetric.value, PluginMetric.recorded_at)
+        .where(
+            PluginMetric.source_module == "host_agent",
+            PluginMetric.metric_name == "cpu_pct",
+            PluginMetric.resource_name.in_(host_names),
+            PluginMetric.recorded_at >= cutoff,
+        )
+        .order_by(PluginMetric.recorded_at)
+    )).all()
+    by_host: dict[str, list[float]] = {}
+    for r in rows:
+        by_host.setdefault(r.resource_name, []).append(r.value)
+    # Amber line to read as "CPU"; only draw when there are a couple of points.
+    return {
+        name: sparkline_svg(vals[-40:], width=64, height=18, stroke="#c8a840")
+        for name, vals in by_host.items() if len(vals) >= 2
+    }
+
+
 @hosts_bp.get("/overview/widget")
 @require_role(UserRole.viewer)
 async def overview_widget():
@@ -180,12 +210,14 @@ async def overview_widget():
                 & (DnsResult.resolved_at == latest_dns_subq.c.max_at),
             ))).scalars()}
         uptime = await _compute_uptime(db)
-        agent = await _agent_overview_by_host(db, [h.name for h in hosts])
+        host_names = [h.name for h in hosts]
+        agent = await _agent_overview_by_host(db, host_names)
+        cpu_sparks = await _agent_cpu_sparks_by_host(db, host_names)
 
     return await render_template(
         "hosts/overview_widget.html",
         hosts=hosts, latest_pings=latest_pings, latest_dns=latest_dns,
-        uptime=uptime, agent=agent,
+        uptime=uptime, agent=agent, cpu_sparks=cpu_sparks,
     )