added 'cleanup_big_files' function to manyfold.sh.
Demo:
```
$ docker run xrforge
...
````
create a 10MB dummy_file.txt (and rewind date some years):
```
$ docker exec xrforge 'dd if=/dev/zero of=/mnt/models/dummy_file.txt bs=1M count=10'
10+0 records in
10+0 records out
10485760 bytes (10.0MB) copied, 0.005670 seconds, 1.7GB/s
$ touch -d "2021-01-01" /mnt/models/dummy_file.txt
$
```
Now when setting maxmb's to `5` and maxdays to `5`, we can see in the logs
that it gets deleted:
```
[/bin/infinite cleanup_big_files /mnt/models 5 5] Fri Aug 1 14:19:08 UTC 2025 executing
deleting: /mnt/models/dummy_file.txt
```
This behaviour is default, and can be disabled by docker environment-flag `NO_DISABLEBIGFILES`
