feat(alerts): group related alerts by resource hierarchy on overview

Alerts that share a resource ancestor (e.g., ZFS pool + device within
that pool) are now grouped on the alerts overview page. The primary
alert (highest severity, most recent) is shown by default with a
'+N related' toggle that expands to reveal the related alerts.

Grouping uses resourceId nesting: alerts with 3+ path segments are
sub-components and share the group key of their parent (first 2
segments). Alerts on different subsystems remain separate.

Single-alert groups render exactly as before — no visual change when
there's nothing to group.

frontend-modern/scripts/canonical-platform-audit.mjs

@@ -31,6 +31,7 @@ const ALLOWLIST = new Set([
   'src/components/Storage/storageSourceOptions.ts',
   'src/components/Storage/useStorageExpansionState.ts',
   'src/components/Storage/useStorageResourceHighlight.ts',
+  'src/features/alerts/useAlertGroupExpansion.ts',
   'src/components/Infrastructure/resourceBadges.ts',
   'src/components/Settings/reportingResourceTypes.ts',
   'src/utils/canonicalResourceTypes.ts',

frontend-modern/src/features/alerts/AlertOverviewActiveAlertsSection.tsx

@@ -12,6 +12,7 @@ import {
 } from '@/utils/alertOverviewPresentation';
 
 import { AlertOverviewAlertCard } from './AlertOverviewAlertCard';
+import { useAlertGroupExpansion } from './useAlertGroupExpansion';
 import type { AlertIncidentTimelineState } from './useAlertIncidentTimelineState';
 import type { AlertOverviewState } from './useAlertOverviewState';
 
@@ -25,6 +26,7 @@ interface AlertOverviewActiveAlertsSectionProps {
 }
 
 export function AlertOverviewActiveAlertsSection(props: AlertOverviewActiveAlertsSectionProps) {
+  const { isGroupExpanded, toggleGroup } = useAlertGroupExpansion();
   return (
     <div>
       <SectionHeader title={getAlertOverviewActiveSectionTitle()} size="md" class="mb-3" />
@@ -116,13 +118,41 @@ export function AlertOverviewActiveAlertsSection(props: AlertOverviewActiveAlert
               {getAlertListEmptyState(props.showAcknowledged)}
             </div>
           </Show>
-          <For each={props.state.filteredAlerts()}>
-            {(alert) => (
-              <AlertOverviewAlertCard
-                alert={alert}
-                state={props.state}
-                timelineState={props.timelineState}
-              />
+          <For each={props.state.groupedAlerts()}>
+            {(group) => (
+              <div>
+                <AlertOverviewAlertCard
+                  alert={group.primary}
+                  state={props.state}
+                  timelineState={props.timelineState}
+                />
+                <Show when={group.related.length > 0}>
+                  <div class="ml-4 border-l-2 border-border pl-3 mt-1">
+                    <button
+                      type="button"
+                      class="text-xs text-muted hover:text-base-content transition-colors py-1"
+                      onClick={() => toggleGroup(group.key)}
+                    >
+                      {isGroupExpanded(group.key)
+                        ? 'Hide'
+                        : `+${group.related.length} related`}
+                    </button>
+                    <Show when={isGroupExpanded(group.key)}>
+                      <div class="space-y-2 mt-1">
+                        <For each={group.related}>
+                          {(alert) => (
+                            <AlertOverviewAlertCard
+                              alert={alert}
+                              state={props.state}
+                              timelineState={props.timelineState}
+                            />
+                          )}
+                        </For>
+                      </div>
+                    </Show>
+                  </div>
+                </Show>
+              </div>
             )}
           </For>
         </div>

frontend-modern/src/features/alerts/useAlertGroupExpansion.ts

@@ -0,0 +1,18 @@
+import { createSignal } from 'solid-js';
+
+export function useAlertGroupExpansion() {
+  const [expandedGroups, setExpandedGroups] = createSignal<Set<string>>(new Set());
+
+  const toggleGroup = (key: string) => {
+    setExpandedGroups((prev) => {
+      const next = new Set(prev);
+      if (next.has(key)) next.delete(key);
+      else next.add(key);
+      return next;
+    });
+  };
+
+  const isGroupExpanded = (key: string) => expandedGroups().has(key);
+
+  return { isGroupExpanded, toggleGroup };
+}

frontend-modern/src/features/alerts/useAlertOverviewState.ts

@@ -5,6 +5,21 @@ import type { Alert } from '@/types/api';
 import type { Override } from './types';
 import { useAlertAcknowledgementState } from './useAlertAcknowledgementState';
 
+export interface AlertGroup {
+  key: string;
+  primary: Alert;
+  related: Alert[];
+}
+
+export function computeAlertGroupKey(alert: Alert): string {
+  const rid = alert.resourceId ?? '';
+  const segments = rid.split('/');
+  if (segments.length > 2) {
+    return segments.slice(0, -1).join('/');
+  }
+  return rid;
+}
+
 export interface UseAlertOverviewStateProps {
   activeAlerts: Accessor<Record<string, Alert>>;
   overrides: Accessor<Override[]>;
@@ -66,9 +81,29 @@ export function useAlertOverviewState(props: UseAlertOverviewStateProps) {
       }),
   );
 
+  const groupedAlerts = createMemo<AlertGroup[]>(() => {
+    const sorted = filteredAlerts();
+    const groups = new Map<string, Alert[]>();
+    for (const alert of sorted) {
+      const key = computeAlertGroupKey(alert);
+      const existing = groups.get(key);
+      if (existing) {
+        existing.push(alert);
+      } else {
+        groups.set(key, [alert]);
+      }
+    }
+    const result: AlertGroup[] = [];
+    for (const [key, alerts] of groups) {
+      result.push({ key, primary: alerts[0], related: alerts.slice(1) });
+    }
+    return result;
+  });
+
   return {
     alertStats,
     filteredAlerts,
+    groupedAlerts,
     unacknowledgedAlerts,
     processingAlerts,
     bulkAckProcessing,