feat(hosts): notification severity and trend charts on metrics tab

[simonjcarr]

Summary

Adds host-scoped notification charts to Monitoring → Metrics on every host detail page.

New charts

Chart	Description
Notification Severity	Donut chart — critical / warning / info breakdown for this host only
Notification Trend	Line chart — critical & warning counts over a selectable window (1h → 3 months)

Both charts sit below the existing Heartbeat Interval chart in the metrics tab.

Implementation

• New component host-notification-charts.tsx in the host detail directory — self-contained, owns its own time-range state and queries
• getNotificationStats and getNotificationsOverTime now accept an optional hostId parameter; when provided they add resource_type = 'host' and resource_id = hostId to the WHERE clause — existing callers (global notifications page) pass no hostId and are unaffected
• Charts use the same currentUserId context as the rest of the host page so results are scoped to the viewing user's notifications about this host
• Trend chart includes the same time-range dropdown (1h · 6h · 12h · 24h · 7d · 30d · 3 months) with hourly/daily aggregation and dynamic subtitle as the global notifications page

Test plan

• Open a host detail page → Monitoring → Metrics
• Pie chart renders with correct severity colours and percentages
• Trend chart renders with legible axis labels in dark mode
• Changing the trend range dropdown fetches and displays the correct data
• Both charts show "No notifications / No data" placeholder when no notifications exist for this host
• Global notifications page charts are unaffected
• pnpm run build passes with no TypeScript errors

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