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/dashboard/routes.py

@@ -173,7 +173,7 @@ async def _get_panels_data(db, dashboard_id: int) -> tuple:
     result = await db.execute(select(Dashboard).where(Dashboard.id == dashboard_id))
     dash   = result.scalar_one_or_none()
     if dash is None:
-        return None, [], []
+        return None, [], [], []
     widgets     = await _get_widgets(db, dashboard_id)
     plugins_cfg = current_app.config.get("PLUGINS", {})
     available   = get_available_widgets(plugins_cfg)
@@ -182,15 +182,17 @@ async def _get_panels_data(db, dashboard_id: int) -> tuple:
         {"db": w, "defn": WIDGET_REGISTRY[w.widget_key], "title": w.title or WIDGET_REGISTRY[w.widget_key]["label"]}
         for w in widgets if w.widget_key in WIDGET_REGISTRY
     ]
-    return dash, resolved, available
+    # Hosts feed the "host" param dropdown (e.g. the history-graph widget).
+    hosts = list((await db.execute(select(Host).order_by(Host.name))).scalars())
+    return dash, resolved, available, hosts
 
 
 async def _panels_response(dashboard_id: int):
     async with current_app.db_sessionmaker() as db:
-        dash, resolved, addable = await _get_panels_data(db, dashboard_id)
+        dash, resolved, addable, hosts = await _get_panels_data(db, dashboard_id)
     return await render_template(
         "dashboard/_edit_panels.html",
-        dashboard=dash, widgets=resolved, addable=addable,
+        dashboard=dash, widgets=resolved, addable=addable, hosts=hosts,
     )
 
 
@@ -369,12 +371,12 @@ async def edit(dash_id: int):
     user_id   = current_user_id()
     user_role = session.get("user_role", "viewer")
     async with current_app.db_sessionmaker() as db:
-        dash, resolved, addable = await _get_panels_data(db, dash_id)
+        dash, resolved, addable, hosts = await _get_panels_data(db, dash_id)
     if dash is None or not _can_edit(dash, user_id, user_role):
         abort(403)
     return await render_template(
         "dashboard/edit.html",
-        dashboard=dash, widgets=resolved, addable=addable,
+        dashboard=dash, widgets=resolved, addable=addable, hosts=hosts,
     )