Bug 503379

SUMMARY
When tagging many images from the thumbs view the GUI becomes non responsive (spinwheel) for minutes(!) while macOS indicates the application is non responsive as well.
At some point the `process indicator` will show it is busy with "background" tasks, shortly removing the spinwheel, which will return immediately until an internal state is reached that the GUI becomes responsive again.

STEPS TO REPRODUCE
1. Select number of images (100 or so)
2. Select a tag from Captions/Tag side menu
3. Apply
4. GUI spinwheel shown / no updates on active processes / macOS shows unresponsive app in the right mouse application menu on the taskbar and/or `activity monitor` app. Can take many minutes.
5. At some point spinning wheel disappears and active processes information is shown, indicating ongoing processes
6. Spinning wheel may appear again and GUI freezes again
7. GUI may show 'hick-ups' with disappearing and reappearing of the spinner until the application becomes fully available again

EXPECTED RESULT
Being able to assign (many) tags without the GUI freezing and continue working while the background tasks (`applying metadata` & `writing metadata to files`) should be really background tasks which do not interfere with the UI.

SOFTWARE/OS VERSIONS
macOS 15.4.1 - M2 Max with 64GB RAM
Digikam 8.6.0 - with Writing XMP side-cars enabled
Ubuntu NAS server with SMB. Approx 250K JPG/CRW/CR2/HEIC images
MariaDB 11.8.1 on Debian12