Fix memory usage on integrity check

[yurabakhtin]

What kind of change does this PR introduce? (check at least one)

• Bugfix

The PR fulfills these requirements:

• All tests are passing
• Changelog was modified

Other information:
Fix similar issue as humhub/cfiles#198

[what-the-diff]

PR Summary

• Occasional High Memory Usage Resolved
  A fix has been implemented to address the issue of high memory consumption when running integrity checks.
• Efficient File Deletion Mechanism
  Changes have been made in the way files are deleted to ensure the process runs smoothly even if the target file doesn't exist. The code now iterates over each file individually.
• Improved Post Deletion Function
  Just as with files, deleting a post when its content record doesn't exist will no longer cause problems. The system now efficiently handles each post in isolation during the deletion process.