…kups

Implement flock()-based serialization to ensure only one backup runs at a time.
This prevents OOM killer (exit code 137) when multiple backups are triggered
simultaneously by EasyDash, as each backup calculates available RAM independently.

Problem:
When 5 backups trigger at once, each checks available RAM (e.g., 8GB) and
allocates buffers accordingly. Total memory = 5 × 8GB = 40GB → OOM killer.

Solution:
Global lock ensures backups run sequentially. Each backup gets accurate
RAM reading and full system resources.

Implementation details:
- Uses PHP flock() for atomic, race-condition-free locking
- Waiting backups poll every 30 seconds with status messages
- Maximum wait time of 2 hours before timeout (error code 5001)
- Lock file stores current backup site URL and PID for debugging
- Shutdown handler ensures lock release on any exit (error, crash, kill)
- release_global_backup_lock() is idempotent (safe to call multiple times)

Lock file location: EE_ROOT_DIR/services/backup-global.lock

Error codes:
- 5001: Timeout waiting for another backup to complete
- 5002: Cannot create backup lock file

Move backup-global.lock from EE_ROOT_DIR/services/ to EE_BACKUP_DIR/
to keep all backup-related files in one location.

Lock file location: EE_BACKUP_DIR/backup-global.lock