Adds freshness metadata tracking for all monitored instances:
- StalenessTracker with per-instance last success/error/mutation timestamps
- Change hash detection using SHA1 for detecting data mutations
- Normalized staleness scoring (0-1 scale) based on age vs maxStale
- Integration with PollMetrics for authoritative last-success data
- Wired into all poll functions (PVE/PBS/PMG) via UpdateSuccess/UpdateError
- Connected to scheduler as StalenessSource implementation
Task 4 of 10 complete. Ready for adaptive interval logic.
- Add comprehensive test coverage for alerts package with 285+ new tests
- Implement ThresholdsTable component with metric thresholds display
- Enhance Alerts page UI with improved layout and metric filtering
- Add frontend component tests for Alerts page and ThresholdsTable
- Set up Vitest testing infrastructure for SolidJS components
- Improve config persistence with better validation
- Expand discovery tests with 333+ test cases
- Update API, configuration, and Docker monitoring documentation
