Proxmox reports cumulative byte counters that update unevenly across
polling intervals, causing a steady 100 Mbps download to appear as
spikes up to 450 Mbps in sparkline charts. Replace per-interval rate
calculation with a 4-sample sliding window (30s at 10s polling) that
averages over the full span — the same approach Prometheus rate() uses.
MetricsHistory.Cleanup() was defined but never called, and even if called,
it only removed old data points without deleting map entries for deleted
containers/VMs. Each stale entry leaked ~224KB (7 pre-allocated slices).
Changes:
- Call metricsHistory.Cleanup() and rateTracker.Cleanup() in maintenance loop
- Delete map entries entirely when all data points have expired
- Return nil instead of empty slice in cleanupMetrics() to release backing arrays
- Add Cleanup() method to RateTracker with 24-hour stale threshold
- Add debug logging to track cleanup activity
Related to #1153
