feat(15-03): add alert rules UI, alert events table, and notification bell

- AlertRuleFormDialog with rule type selector, threshold input, auto-set units
- AlertRulesTab with list, enable toggle, edit, delete, and add button
- AlertEventsTable with severity badges, resolve action, and state filter tabs
- NotificationBell polls active alert count with 60s interval
- Site dashboard gains Alerts tab rendering both AlertRulesTab and AlertEventsTable
- NotificationBell integrated into ContextStrip header for tenant users

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/ContextStrip.tsx

@@ -16,6 +16,7 @@ import { useUIStore } from '@/lib/store'
 import { tenantsApi, metricsApi } from '@/lib/api'
 import { useEventStreamContext } from '@/contexts/EventStreamContext'
 import type { ConnectionState } from '@/hooks/useEventStream'
+import { NotificationBell } from '@/components/alerts/NotificationBell'
 
 const SYSTEM_TENANT_ID = '00000000-0000-0000-0000-000000000000'
 
@@ -191,6 +192,9 @@ export function ContextStrip() {
 
       {/* Right: Actions */}
       <div className="flex items-center gap-2 ml-auto">
+        {/* Notification bell */}
+        {tenantId && <NotificationBell tenantId={tenantId} />}
+
         {/* Command palette shortcut */}
         <button
           onClick={() => useCommandPalette.getState().setOpen(true)}