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
- Fix SSRF and rate limit bypass in SendEnhancedWebhook by validating the rendered URL.
- Fix rate limit spoofing in updates API by using secure IP extraction (trusted proxies).
- Fix memory leak in metrics history by correctly clearing fully stale data series.
- Fix public URL poisoning by preventing overwrites when explicitly configured.
