nlog: add disk-pressure-aware retention to fix #721

[SBALAVIGNESH123]

Fixes #721

When disk usage reaches 90%, the existing retention policy doesn't kick in because the min_size_to_keep guard (default 4GB) blocks deletion — even if the disk is nearly full, individual logs smaller than 4GB won't have their segments cleaned up. This means retention only works based on per-log size and time thresholds, but never considers actual disk space.

This PR adds a fourth deletion strategy, deleteDiskPressureBreachedSegments, that activates only when disk usage exceeds a configurable threshold (default 90%). When triggered, it bypasses the min_size_to_keep guard and deletes the oldest segments to free disk space. The active segment is always preserved by the existing deletableSegments() mechanism. When disk usage is below the threshold, behavior is completely unchanged.

Changes:

• LogConfig.h / LogConfig.cpp — Added disk_usage_threshold_percent (default 90%) with validation and debug output
• Log.h / Log.cpp — Added deleteDiskPressureBreachedSegments() as a fourth deletion strategy, called only when disk_pressure flag is true
• LogManager.cpp — Added std::filesystem::space() check on each root directory in cleanupLogs() to detect disk pressure