Planify grows in ram usage when syncing with Nextcloud instance

[oktayacikalin]

Describe the bug
Every time the sync with Nextcloud is running, planify grows in ram usage. After some time it is getting around 3-4 GB. Looks like a memory leak.

To Reproduce

1. Configure sync with Nextcloud
2. Inspect current ram usage
3. Click on sync or let it sync automatically
4. Inspect current ram usage
5. Planify has grown in ram usage

Expected behavior
Planify should not constantly grow in ram usage.

Screenshots
Video showing the memory leak in action:
https://github.com/user-attachments/assets/ce462b40-74fb-48c4-9eab-d4efa412ab3e

Desktop (please complete the following information):

• Fedora Workstation 41 with GNOME 47
• Planify 4.11.6 via flathub

Additional context
Nothing logged into the console:
https://github.com/user-attachments/assets/8211d734-7836-45e1-bb34-e88171b8b793

When syncing only a particular list, the leak seems to be related to the number of entries it contains. For example "Persönlich" leaks about 4 MB per sync.